Most sources I find seems to portray women's participation in warfare, as leaders and or soldiers, as more of an exception than the rule during antiquity and the middle ages.
Is there any source that gives evidence of women in fighting roles during this period? Were there armies known to enroll women in significant numbers?
SHORT ANSWER
In antiquity, the Scythians (Eurasian nomads) and the Sarmatians (nomads of Iranian origin who moved westwards, gradually overwhelming the Scythians) had significant numbers of female warriors. Estimates based on archaeological discoveries range from 15% to over 30% of women who were warriors. The precise role of these fighters remains unclear, as do the reasons why these related cultures had female warriors while other, similar, cultures apparently did not. Note:'Scythians' is sometimes used to include 'Sarmatians' while at other times it refers only to the people west of the Don River. This, as Wikipedia notes, has led to a fair amount of confusion.
MAIN ANSWER
Archaeological finds, with the aid of science, over the last 25 years have provided an increasing amount evidence that Scythian women and those of their eastern kin the Sarmatians (among whom the Sauromatae are most frequently mentioned) fought in significant numbers, and that the writings of Herodotus and other ancient historians on this subject have at least some basis in fact. Earlier finds have also been reassessed as it was previously assumed that any grave containing weapons belonged to a male. Deborah Levine Gera, Professor of Classics at the Hebrew University of Jerusalem, in Warrior Women: The Anonymous Tractatus De Mulieribus asserts:
The presence of...Scythian women warriors in the pages of Herodotus and Ctesias reflects some kind of historical reality, for there is evidence that some Scythian women rode on horseback, used bows and arrows, and went into battle. Archaeological remains indicate that there were female Scythian warriors, chiefly - but not solely - among the Sauromatae,..
This Sarmatian female warrior tomb "was found with more than 100 arrowheads, a horse harness, a collection of knives and a sword". Source: ZME Science
In The Scythians 700-300 BC by the archaeologist Dr. E. V. Cernenko, the author asserts:
Almost the whole of the adult population of Scythia, including a large number of the womenfolk, fought on campaign.
What evidence is there for the above, other than Herodotus? For the archaeological evidence, the Smithsonian article The Amazon Women: Is There Any Truth Behind the Myth? from 2014 relates the following from the early 1990s:
...a joint U.S.-Russian team of archaeologists made an extraordinary discovery while excavating 2,000-year-old burial mounds—known as kurgans...outside Pokrovka...near the Kazakhstan border. There, they found over 150 graves belonging to the Sauromatians and their descendants, the Sarmatians. Among the burials of “ordinary women,”... There were graves of warrior women who had been buried with their weapons. One young female, bowlegged from constant riding, lay with an iron dagger on her left side and a quiver containing 40 bronze-tipped arrows on her right. The skeleton of another female still had a bent arrowhead embedded in the cavity....On average, the weapon-bearing females measured 5 feet 6 inches, making them preternaturally tall for their time.
The last sentence is interesting as it deals with the perception that women are physically much less well equipped to fight compared to men; these women seem to have been an exception. It is also worth considering that a child, male or female, brought up from a young age to ride horses and use a bow and arrow, is likely to become a formidable adversary.
The Smithsonian article continues with
In recent years, a combination of new archaeological finds and a reappraisal of older discoveries has confirmed that Pokrovka was no anomaly.
According to Kathryn Hinds in Scythians and Sarmatians, the Pokrovka graves were those of "ordinary people" (not royalty), and following figures give an idea of the percentage of female warriors, at least within one community:
The vast majority of the men — 94 percent — were buried with weapons...
15 percent of the women were warriors, buried with arrowheads and other weapons.
Other discoveries suggest a higher percentage. This article from the Irish Times seems to refer to a more recent find:
A team of archaeologists investigating 2,400-year-old burial mounds built by the Scythian people on the upper reaches of the river Don has found that five of 21 graves contained the bodies of young women, accompanied by their weapons.
The article also quotes Dr Valery Gulyayev, of the Russian Institute of Archaeology:
"Usually such women are found in large kurgans, buried with the same rituals as for men," ... "They are buried with womanly things - mirrors of silver and bronze, necklaces of gold, glass or clay, earrings. But alongside these they are buried with weapons - a quiver, bow and arrows, and, often, two throwing spears.
The National Geographic article Amazon Warriors Did Indeed Fight and Die Like Men cites archaeological finds of Scythians which have been subjected to scientific testing:
Archaeologists have found skeletons buried with bows and arrows and quivers and spears and horses. At first they assumed that anyone buried with weapons in that region must have been a male warrior. But with the advent of DNA testing and other bioarchaeological scientific analysis, they've found that about one-third of all Scythian women are buried with weapons and have war injuries just like the men. The women were also buried with knives and daggers and tools.
Unfortunately, the article doesn't say which specific find the above relates to, but there is again some evidence here of 'significant numbers'. The New Yorker article The Real Amazons, citing Adrienne Mayor, research scholar in the Classics Department at Stanford University, says:
...in some archaeological digs in Eurasia, as many as thirty-seven per cent of the graves contain the bones and weapons of horsewomen who fought alongside men. (“Arrows, used for hunting and battle, are the most common weapons buried with women, but swords, daggers, spears, armor, shields, and sling stones are also found,” Mayor writes.)
In the light of the archaeological evidence, there has unsurprisingly been some reassessment of ancient sources. Further, it should be noted that Herodotus was not the only writer to refer to female warriors; there is also Ctesias, Hippocrates (see the passage cited here) and - as J Asia mentioned in his comment - Diodorus. They have embellished in parts and got some details badly wrong, but the archaeological evidence seems to support the assertions of a significant number of female warriors among some of the Scythians and some of their kin, perhaps most notably the Sauromatians.
Role of female warriors
The precise role of female warriors is unclear but it is most likely that they (1) defended the community while the main body of fighting men were away, and (2) were 'called up' and fought alongside men in times of great need, such as the Persian invasion under Darius I (ruled 522 to 486 BC). Their involvement in the army may well have gone beyond these, but archaeology has yet to conclusively prove this.
Reasons for significant number of female warriors
Also unclear is why these two cultures had significant numbers of female warriors while others around them apparently did not. None of the sources cited here deal with this directly, but several suggest that the prominent role women played as rulers is significant. For example, one archaeological dig found that over 70 % of the central graves (i.e. those of highest status) had female remains.
David W. Anthony, in The Horse, the Wheel and Language, notes an interesting point about the Scythians and the Yamna people "dating to 3300–2600 BC" who were there much earlier,
About 20% of Scythian - Sarmatian "warrior graves" on the lower Don and lower Volga contained females dressed for battle as if they were men...It is at least interesting that the frequency of adult females in central graves under Yamnaya kurgans in the same region, but two thousand years earlier, was about the same. Perhaps the people of this region customarily assigned some women leadership roles that were traditionally male.
This, though, is only the beginning of an answer, but we can also consider the prominent role played by archers on horseback where physical strength (though not unimportant) plays less of role than it would in close-quarters combat (as suggested by orangesandlemons in his comment). We should also not forget that in any society, there are always some women who are physically stronger than some men. A further point worth mentioning is that there may have been much less gender distinction in the division of labour than in other cultures; this is in evidence among modern Kazakh nomads where boys and girls compete directly "in riding exercises and games" (DNA testing showed that one girl had the same common ancestor as a 5th or 4th century BC female warrior buried at Pokrovka).
All-female society and Thracian female warriors
Two further points worth commenting on are first, the claims (particularly in Herodotus) concerning an all-female warrior society and second, claims that there were significant numbers of female Thracian warriors. On the former,
As yet, Davis-Kimball (2002) notes that there is no archeological evidence linking all the storied versions (e.g., no excavated settlement suggests that women warriors lived in societies without men).
On the latter, there is a lack of sufficient evidence, as demonstrated in Fingerprinting the Iron Age (Nicolae Popa, Simon Stoddart, eds.). However, Women in Antiquity (Stephanie Lynn Budin, Jean Macintosh Turfa, eds.) does cite one interesting discovery.
Other sources:
Hamid Wahed Alikuzai, A Concise History of Afghanistan in 25 Volumes, Volume 14
The excellent answer above by Lars Bosteen details a significant exception to the case outlined below. Perhaps additional exceptional instances remain to be uncovered in the historical and archaeological records.
Let's start by parsing the question actually asked; rather than imagining a different question that might appeal.
Is there any evidence of armies enrolling women in fighting roles in significant number in ancient history and middle ages?
Note a few key phrases here:
enrolling
fighting roles
significant number
ancient history or middle ages
To answer this question in the affirmative, it will be necessary to satisfy the criteria specified by these key phrases: That evidence exists that armies enrolled women, in fighting roles, in significant number, during the period of ancient history or the middle ages.
To take these key phrases out of order:
1. ancient history or middle ages
A common end-date for the middle ages is 1500. That works for me. It also roughly marks when warfare (in Europe, which would soon dominate the world for several centuries) becomes dominated by firearms and artillery rather than melee weapons.
The Zulu don't exist as a nation until the late 18th century (and very little is known of the Nguni people prior to 1500) so mentions of 19th century Zulu amazons are irrelevant to the question. So are the (relatively sparse but existing) accounts of women participating in the Napoleonic Wars, including accounts of Massena's mistress. The relevant time period here must approximate 3000 B.C.E. to 1500 C.E. to satisfy the question.
2. significant number
A normal measure of significance is 5%, or one in twenty. That's a lot of women, to comprise 5% of your fighting roles with them. There is simply no evidence in any historical record of 5% of soldiers being women. Such evidence is in fact conspicuous by its absence. This does not mean that no women ever participated in combat; it means that there is no record, anywhere in the relevant historical record, of cases where women were 5% or more of deliberately enrolled combatants.
3. enrolling
Satisfying this criteria requires that the relevant armies had a proactive policy of recruiting suitable women, in significant number, for combat roles. Again, the entire relevant historical record is absent of any such evidence.
Let's also distinguish between armies and garrisons. From Troy to the Alamo, the fate of besieged once a breach is made has generally been a sad tale of fire, rape, murder and pillage. Of course every breathing adult has been armed to the teeth, in hope of taking as many besiegers to the grave as possible. That does not count as enrolling in an army combat role - it's just desperation.
4. fighting roles
This eliminates camp followers whose primary task is logistical in nature, whether wife, mistress, cook, nurse, etc. We are talking tooth and claw end of the army here, not its tail. Again, no evidence in the historical record to satisfy this.
What evidence do we have for women in combat during this period? A few, notable in their rarity:
Boudicca - who certainly commanded but may not have actually fought.
Jeanne d'Arc - who certainly did fight as well as command.
Several more examples of Warrior Queens are given here in the eponymous section about 3/4 down, the bulk of whom were NOT actual combatants.
Undoubtedly a number more, who are not coming to mind right now. But the very fact that we know all their names speaks to the extreme rarity of their occurrence; and in no instances was there ever an enrolment process for women into combat roles in these armies.
Absence of evidence is not evidence of absence ...
... but if you are looking for modern Western type of politically correct army, you are not going to find it in antiquity. No nation at that time would send child-bearing young women to be slaughtered in a role that ill suited them. Unlike us, ancients were not so deluded to pretend that upper body strength and overall mass do not count in warfare. And remember, these were the times of archers and lancers.
However, there is some evidence for female rear guard troops ...
...not very glamorous duty, but when every available male had to go to war, often woman would have to protect rear areas. Scythian warrior women were never equal to men (except in modern feminist propaganda :) ) but they did have significant role as auxiliary troops :
"Yes, there probably was an obligation on the women to serve as warriors," he said. "But it seems likely that when the men left the settlements to pasture their herds, they left the women on guard. These young women and girls on horseback were in the role of lightly armed troops. They were guarding the hearth and the homestead."
African women have a long legacy of being rulers of nation states, generals of armies, warriors, and leaders of rebellions for thousands of years; several examples include Hatshepsut, pharoah of Ancient Egypt (1507-1458 B.C.E.); Amanirenas, Kandake of the Meroitic Kingdom of Kush (60-50 B.C.E.-10 B.C.E.); Queen Nanny of the Maroons (c. 1686- c. 1755; born in Ghana; a Jamaican National Hero); Amazons of Dahomey (c. 1685-1892, Kingdom of Dahomey (1600-1892), present day Republic of Benin, "they numbered between 1,000 and 6,000 women, about a third of the entire Dahomey army"; see Seh-Dong-Hong-Beh "In 1851, she led an all-female army consisting of 6,000 warriors against the Egba fortress of Abeokuta, to obtain slaves from the Egba people for the Dahomey slave trade"); and in recent history Harriet Tubman (c. 1822-March 10, 1913).
