This is where application pays off. It’s much more difficult to learn something if you never apply it. Application actually creates mental bandwidth that can be used to learn other things. Because these athletes apply their skills, they are not wasting energy in a training loop.
Because I drive a stick shift every day, I don’t have to continuously keep learning how to drive.
Simply put, if you aren’t learning the skills so that you can do them, maybe one system is the max. Ninja, karate, aikido... maybe one at a time. Systems where you use them for some purpose, I would say more than one is very possible. Baseball, football, basketball, judo, wrestling, BJJ, judo, kyokushin karate, chess, clarinet... you could probably learn any three or four before you run out of time.