As a rule of thumb, "they" say we should have 1 more litterbox than number of cats, just to be on the safe side. I concur, and presently have 3 cats and 4 boxes. Meeshe (FeLV+) is kept separate, so really he has his own. Lily and Debra have 3 between them, but Debra is a new rescue and the two girls are not free to interact with each other just yet. So Lily has her one, and Debra has two because she is just a baby girl.

I maintain about 4 inches of scoopable litter in each deep box, and NEVER have to completely change the litter; I just add a mug-full as needed. I always scoop morning and evening, because a clean box is a useful box, whereas a dirty box is . . . well . . . only inviting the cat to find another toilet!