Which is more correct, Carlos I or Carlos V?

In his role as Spanish King, including Spanish possessions

• Charles I, King of Spain

otherwise (in Pan-European context)

• Charles V, Holy Roman Emperor

Since he was active in both areas, the different titles should be used in context.

For persons familiar with British history, Charles V and Charles I are 2 distinct persons, both of which are well known during the Tudor and Stuart periods.

So usage without dates or the 'of Spain' portion may cause confusion.

Charles V (24 February 1500 – 21 September 1558) was Holy Roman Emperor and Archduke of Austria from 1519, King of Spain (Castile and Aragon, as Charles I) from 1516, and Lord of the Netherlands as titular Duke of Burgundy from 1506. Head of the rising House of Habsburg during the first half of the 16th century, his dominions in Europe included the Holy Roman Empire extending from Germany to northern Italy with direct rule over the Austrian hereditary lands and the burgundian Low Countries, and a unified Spain with its southern Italian kingdoms of Naples, Sicily, and Sardinia. Furthermore, his reign encompassed both the long-lasting Spanish and short-lived German colonizations of the Americas. The personal union of the European and American territories of Charles V was the first collection of realms labelled "the empire on which the sun never sets".

Sources:

• Charles V, Holy Roman Emperor
• Charles I of England

There is a long discussion about the article name in Wikipedia in Spanish. In spite of being a heated and probably un-scientific debate, it shows that both names are used. A rapid check of how are streets named after the emperor (by googling "calle Carlos I" and "calle Carlos V") shows that both are used.

Furthermore, neither Charles I nor Charles V are ambiguous in a Spanish context, since there is just another king named Charles (Charles II). This might be a bit different in Navarre where up to four Charles reigned, but in most contexts references to Charles I can't be ambiguous with a 14th century king of Navarre.

In summary, I wouldn't worry because both names can be unambiguously understood and are used often.

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.

https://www.reddit.com/r/history/comments/dwzp2h/monarchs_with_different_numbers/1

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.