Wait
But if all the magisters have that body hopping power.
Won't they be impossible to kill?
They won't all be as dumb as Cory as to put some of their power in a dragon so they can't use it for a bit once it's dead.
So please let Cory be the only with it. Otherwise the amount of stupid necessary to be able to kill the rest of them might make my brain explode.
Gaider has said that the Architect "obviously" won't be back if you killed him. And while Warder is right that Gaider hasn't outright said the Architect is not a former Magister Gaider's word is a better description of that than it is of his alternative. I take that to mean that there is at least one former Magister that cannot bodyhop. Unless Gaider changes his mind, which I'd prefer he not do once he's established something through a source that cannot be reasonably impeached.
Another thing, we know seven magisters went into the Fade. Are we absolutely sure that seven came back out?
I'd been assuming they all did, but the answer to your question is "no." Furthermore there's a Codex that implies that they fought among themselves and that not all of them survived that. I'm not going to call that canon since it's been made clear the Codex doesn't have all the answers, but it's yet more reason to say that we can't be sure there's a total of seven (or six, since what happened to Cory is almost certainly a death sentence) left.