I know decimation happened a lot in the Roman army, but I find it hard to imagine generals like Marius, Scipio and Caesar, who all had the unwavering loyalty of their troops, practicing it.
In the time of late Republic the decimation was not practised anymore, except the well-known incident on Spartacus uprising:
Five hundred of them, moreover, who had shown the greatest cowardice and been first to fly, he divided into fifty decades, and put to death one from each decade, on whom the lot fell, thus reviving, after the lapse of many years, an ancient mode of punishing the soldiers. For disgrace also attaches to this manner of death, and many horrible and repulsive features attend the punishment, which the whole army witnesses.
Plutarch, "The Life of Crassus", 10
But in the time of early empire the decimation seemed to get in use again:
On receiving this information, Lucius Apronius, successor to Camillus, alarmed more by the dishonour of his own men than by the glory of the enemy, ventured on a deed quite exceptional at that time and derived from old tradition. He flogged to death every tenth man drawn by lot from the disgraced cohort. So beneficial was this rigour that a detachment of veterans, numbering not more than five hundred, routed those same troops of Tacfarinas on their attacking a fortress named Thala.
Tacitus "Annals", III, 21
Are we sure that neither Marius, nor Scipio, nor Caesar practised the decimation? Well, it was still an exceptional measure, and Roman historians should have mentioned it, if it really had taken the place. Besides, the decimation was intended to punish the whole cohort (500 men), so it could be about cowardice in big battles only. And the sources seem to be quite scrupulous when talking about measures taken by Roman commanders after any of big defeats.
It seems to have been rare to the point of never happening. In this Wiki quote, Livy gives one example in 471 BC, which is almost into the legendary times of Roman History. Polybius in the 150 BC timeframe notes the threat, but does not give any instances, even in the disasters of the Punic Wars. Even the losers at Cannae were not decimated, but just forced to stay in the army permanently.
Crassus' decimation is well reported primarily because of how incredible it was to occur at all.
Augustus: the line in Seutonius is "If any cohorts gave way in battle, he decimated them". But no actual examples are given. This is in a paragraph giving a lot of severe acts he did to the army, but did one really happen? There is no description of who, and when, that really convinces.
Galba: Galba's decimation was of a group of Marines that Nero had tried to promote to legionary status. Galba might have not thought these were proper soldiers at all.
So in 1000 years of history we have five instances - 471 BC, Crassus, Antony, Galba in 68 AD, and Lucius Apronius in 20 AD. And Galba might argue that his was not of true Roman soldiery.
So decimation was not common at all in practice.
The earliest documented decimation occurred in 471 BC during the Roman Republic's early wars against the Volsci and is recorded by Livy. In an incident where his army had been scattered, consul Appius Claudius Sabinus Inregillensis had the culprits punished for desertion: Centurions, standard-bearers and soldiers who had cast away their weapons were individually scourged and beheaded, while of the remainder, one in ten were chosen by lot and executed.
Polybius gives one of the first descriptions of the practice in the early 3rd century BC:
If ever these same things happen to occur among a large group of men... the officers reject the idea of bludgeoning or slaughtering all the men involved [as is the case with a small group or an individual]. Instead they find a solution for the situation which chooses by a lottery system sometimes five, sometimes eight, sometimes twenty of these men, always calculating the number in this group with reference to the whole unit of offenders so that this group forms one-tenth of all those guilty of cowardice. And these men who are chosen by lot are bludgeoned mercilessly in the manner described above. The practice was revived by Crassus in 71 BC during the Third Servile War against Spartacus, and some historical sources attribute part of Crassus' success to it. The number of men killed through decimation is not known, but it varies between 1,000 (used on 10,000 men), or a cohort of around 480-500 men, meaning that only 48-50 were killed.
Julius Caesar threatened to decimate the 9th Legion during the war against Pompey, but never did.
Plutarch describes the process in his work Life of Antony. After a defeat in Media:
Antony was furious and employed the punishment known as 'decimation' on those who had lost their nerve. What he did was divide the whole lot of them into groups of ten, and then he killed one from each group, who was chosen by lot; the rest, on his orders were given barley rations instead of wheat. Decimation was still being practised during the time of the Roman Empire, although it was very uncommon. Suetonius records that it was used by Emperor Augustus in 17 BC and later by Galba, while Tacitus records that Lucius Apronius used decimation to punish a full cohort of the III Augusta after their defeat by Tacfarinas in AD 20. G.R. Watson notes that "its appeal was to those obsessed with "nimio amore antiqui moris" – that is, an excessive love for ancient customs – and notes, "Decimation itself, however, was ultimately doomed, for though the army might be prepared to assist in the execution of innocent slaves, professional soldiers could hardly be expected to cooperate in the indiscriminate execution of their own comrades."
There appear to be about 10 or so recorded instances of Decimation, but Decimation is a specific kind of Fustuarium which involves drawing lots instead of just punishing the guilty. There are also variations of it that could involve different levels of punishment like crucifiction or whippings, or lotteries of different odds than 1:10 like Centesimation which can further muddy any concrete attempt to answer this question.
Because these distinctions are not always made by ancient historians and because of the amount of propaganda that existed in the Ancient Roman Empire, it can be hard to say if the exact number of times Decimations were actually done is higher or lower, but I have done my best to compile the following list of probable decimations:
- 471 BCE during the 1st Volscian War (Decimation)
- 315 BCE during the Second Samnite War (Fustuarium that may have been a Decimation)
- 264 to 146 BCE during the Punic Wars (Polybus's historical writings make it sound like it was practiced more than once, but gives no examples.)
- 215 BCE during Second Punic War (Fustuarium that may have been a decimation)
- 71 BCE during the Third Servile War (Decimation)
- 49 to 34 BCE during Roman Civil Wars (Decimation supposedly happened 4 times, but I can't actually confirm this since the cited source is behind a paywall)
- 35 BCE during Antony's Parthian War (Decimation)
- 17 BCE during the Cantabrian Wars (Decimation)
- 20 CE during the Tacfarnius War (Decimation)
- 286 CE peace time punishment (The Theban Legion was decimated repeatedly, then all executed for refusing to observe pagan sacrifices)
^ All dates are approximations, and some sources show slightly different dates for what were probably the same events. I tried not to replicate what may have been duplicates.