We currently have five cats and four litterboxes. We didn't mean to have five. We were originally going for three total. Then we adopted a bonded pair of tortoiseshell sisters, and a year later a bonded pair of tuxedo brothers... oops! So now we have five. That's our maximum, and since our cats range in age from five to two, we count on having them for a while without increasing again. The good side is that there's almost always a lapwarmer available. The downside is four litterboxes.
The most we've ever had was eleven when I was in college. That was an accident - we had a couple of "teenage" kittens and mistakenly believed they had to be at least eight months to be spayed and neutered. At seven months, they decided that being sister and brother was no bar to true love and presented us with four kittens. Then we took in a cat of roughly the same age from a college "friend" who said she was going to dump the cat in the woods. Said cat came to us pregnant. Another four kittens!
We worked with a rescue organization to get the adults and all the kittens spayed and neutered. The group also helped us place the cats. We had found out we needed to move soon so we decided to rehome all of them. I learned a lot about cats then, and it also cemented my love of rescue organizations. I also discovered how tormented female cats in heat really are. It's not a pleasant experience for them. I wouldn't have a cat who wasn't spayed or neutered now.