I usually take care of six of them at the same time. Everyone else usually chills inside the PC and can take care of themselves, since you can only carry six of them at once.

[Ahahah...buying things.]

That depends on the Pokemon! You have to know their diet and how much you need to buy, which can factor into your budget. I also spend some time with them, usually by letting one of them outside of their Pokeball whenever I can.