Emperor Charles V had many realms and many royal and noble titles. So he has different numbers with his name in different Wikipedia lists of monarchs.
As king of the Castilian kingdoms, he was Carlos I.
As king of Aragon, etc., he was Carlos I.
As king of Navarre he was Carlos IV, V, or VI since the list in Wikipedia is messed up. It counts a titular Carlos IV, then the Emperor is listed as Carlos VI, although he could be Carlos IV if the previous Carlos IV isn't counted, then comes King Charles II of Spain listed as Charles V (Charles II of Spain), and King Charles III of Spain is listed as Charles VI again.
As King of Sicily he is listed as Charles or Carlo II.
As King of the other Sicily, or Naples, he is listed as Carlo IV.
As King of Sardinia he is listed as Charles I.
As Duke of Burgundy he is listed as Charles II.
As Count of Burgundy he is listed as Charles II.
As count of Flanders he listed as Charles III.
As Duke of Brabant he is listed as Charles II.
As Archduke of Austria he is listed as Charles I.
And so on and so on. This post says:
HRE Emperor Charles V was actually Charles I in Austria and Spain, Charles II in Burgundy and Hungary, Charles III in Flanders, Charles IV in Sicily.
The positions of King of the East Franks or Germany and King of Italy or Lombardy were united with the position of Emperor in 962, and the position of King of Arles or Burgundy was united with the position of Emperor in 1032. Before those dates the three kingdoms often had different rulers and so they don't always have the name number of rulers with a particular name, like Charles.
As Emperor of the Romans He was Carolus V, as King of Germany he was Karl IV (counting Charlemagne as Karl I), as King of Italy he was Carlo V, and as King of Burgundy he was Charles V.
Note that Emperor Charles V was Count of one Burgundy, Duke of a second Burgundy, and King of a third Burgundy.
Since the Emperor of the Romans claimed to be the rightful ruler of the whole world, it was by far the most important title of Emperor Charles V and so it seems appropriate for him to be identified as Carlos V instead of Carlos I.
The official ideology of the Holy Roman Empire was that it was the continuation of the Carolingian Empire, and that Charlemagne in 800 was the rightful successor of Constantine VI who was deposed in 797 (and replaced by a woman who wasn't eligible to be emperor), and that Constantine VI was the rightful heir of all the eastern Roman Emperors back to Arcadius in 395, who was the heir of all Roman Emperors back to Augustus. And of course everyone knows that the Roman emperors claimed to be the rightful rulers of the world.
When Carlos V is listed or discussed as the ruler of some county, principality, duchy or kingdom, such as Castile, he could be listed as Carlos V (I) or as Carlos I (V) to show that he is known by both numbers.