All of them.
It's hardly all of them. There have always been plenty of Princes that weren't Immortal Elves (and even some that weren't elves at all) -- the "big names" that the fans and writers tend to glom onto, sure. The IE
thing gets certain Princes more page-space than others, so the
feel is too-often one of "ZOMG, every Tir Prince is an IE!" What'd be more accurate, though, would be to say "every Tir Prince that gets much page time is an IE."
And, even then, that's all post-coup attempt. The current Council's still basically one big question mark, other than knowing Zincan, the ork, is High Prince.