When it says 15-turn motor limit for 540 size motors or 12-turn motor limit for 550 size motors. Is that the max # of turn motor you can use or the min # of turns? Thank you for your help

min, the higher the turn # the slower the motor

It means that is the lowest turn motor you can use with the ESC (but not recommended to use the lowest rated motor, it could shorten the lifespan of the ESC).

Is there a limit on how many turn motor you could use the other way? Say a 35t I have a old xl5 sitting around and was wondering if I can use it in a crawler.

no, you can run as high of a turn motor that you want to