Is there a limit on how many focuses I can put on her?

Nope, no limit.

UNLIMITED POWAH!!!

Except for the 'once per turn' clause, of course! One focus token on the Rey-bank at the end of combat; one focus token off Rey and onto the ship at the start of combat. Make sure that Palob has initiative and this becomes immune to his abilty to steal it, too.

You can only place 1 token onto the card per round in the End phase, and you only remove 1 token per round in the Combat phase. But it you don't assign a token during the Combat phase, there's effectively no limit on how many tokens can be stacked up on the card.