"Your Highness"? Not "Your Majesty"?
I'm pretty sure that Highness is for children of the current monarch, Majesty for the monarch. (Though that may be only in England.)
Well, that's my nit-pick for the day.
Anyways, an interesting question, made even more fun by the fact that it, at least used to be, that if a monarch ruled two+ countries they could have two different numbers after their name, which one they used depended on which country they were in at the time.
But I think Galedeep's answered it pretty conclusively.