Well, the recommendation is that you have one litter box per kitty plus an additional one overall. But, it depends on what size your litter box is, the number of trips your kitty visits the box and the regularity of clearing the soiled litter.
When I brought home a kitten recently, I was a little worried that my cat, momo, would not want to share her box. But I was pleasantly surprised by momo's gracious nature.
For hygiene's sake, however, I have started to put out 2 litter boxes side-by-side for the kitties. One medium box and one small one. I clean at least once a day, usually in the morning while they are having breakfast. But it all depends on how soiled the boxes are. Nobody wants to go to a dirty toilet, right? It's disgusting.