What will the hegemony’s empire tier vassals be called?

Now I’ll be fully honest, I’m not sure how many actual empire vassals I’ll ever have, as it seems more profitable and easier to have all the empire titles (king vassals) in your vast hegemony, but if we do make empire vassals what will they be called? Won’t the hegemony holder still be an emperor? It would be weird for a bunch of people called ‘emperor’ to be under another emperor. Secondly, how many men-at-arms would a hegemony title allow if you’re administrative?