Another little known, but interesting, (to ME), fact.
When Queen Elizabeth was given the Regnal Number of 2, the Second, there was some disquiet amongst some Nationalists, in Scotland. who pointed out that in that Ancient Realm, she was The First Elizabeth.
This was taken to the Scottish Courts, where a judge ruled that it was a Royal Prerogative how The Monarch was styled.
Sir Winston, UK PM at the time, and an acknowledged Historian, nailed it with the dictum that where the numbers didn't agree, the highest would be used,
For example, should we ever have a King James, he would not be James 111 but James V111.
If in due course Prince William chooses to use his own name, he will be William V as Scotland has only had three kings with that name previously.
Complicated old world, as if Scotland Becomes Independent, in this case PRESUMABLEY, he would be William 1V in Scotland and William V in England, presuming Scotland retained The Monarchy.