I think it depends on how many you can support. That support includes, but is not limited to, a place to live, food to eat, healthcare, a clean litter box, exercise space, play time, individual time with caretaker, and hiding places for alone time. I have 4, two rescued from the animal shelter and two that found their way to my patio, one starving and one only 6 or 7 weeks old. My cats get along well 99% of the time and for the other 1% I keep a spray bottle of water sitting on the table. All I have to do is pick it up and they scatter and stop the growling, hissing and semi-aggressive behavior. They also share one litter box and we've never had an accident. I use Arm & Hammer Multi-Cat Unscented litter. That's Unscented NOT fragrance free, there a difference and it's a BIG one.
My vet has 26.