At least one litter box for each cat if not more. I have 2 cats and 2 litter boxes. One is bigger than the other and the cats seem to prefer it more, if I had more room I would add another litter box, but don't have the room in a apt. I think it may depend on your cats. Some cats can share litter boxes just fine, but others want their privacy. If you have lots of guests it would be great to put a litter box in a out of the way place and if your cats are always in your bedroom, maybe a litter box is a good idea. Dr Foster and Smith sells these great covers
for boxes. it is a wooden box with carpet in different colors and it has a small opening in the front for the cats. You lift the lid to clean it. It looks really neat.