No, in Spain the regnal numbers follow those of the Kings of Castile.
There were more Carlos or Felipes in Navarre then in Castile, but Kings adopted the Castilian ordinal, for example.
Interestingly, the Carlist claimant Jaime dedided to be “reign” as Jaime III, following the ordinal number of Aragon. There had been two Jaime’s in Aragon, but none in Castile.
About Leonor we do not know. When Kings and Queens are proclaomed with their ordinal number. We will have to wait. 
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I believe that in Spain the highest regnsl number from the former kingdoms is used. Thus, Leonor will be Leonor II.
It’s sensible and I can’t see why it should not be used in Britain. Previous Message
Please also see the original article in The Herald which I linked, and which does make the point that the late Queen chose an approach to this issue which relies on the highest number taken into account from both former kingdoms. Previous MessagePrevious Message
He may become the fifth king of his name to reign over the former kingdom of England and the second king of his name to reign over the former kingdom of Scotland but those kingdoms do not exist anymore. He will be William V of the United Kingdom, in Scotland and in England. The absence of a regnal number in a royal cypher does not mean that the monarch intends to be known as “the First”. He will not be “WIIR”.
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on 10/1/2023, 19:40:17, in reply to "