With a kitten around I always have at least 2 boxes if not 3 boxes. A kitten is not always where the litter box is when he has to go, so best to have another in the othere part of the house. And if the other cat is picky about their box they might not share it. Rule of thumb is one box for each cat and I would also add, one box per levels of your home, ( two story home 2 boxes and if a basement one for there also.) I like to make sure there are enough for them, because I know how I hate for look for the bathroom. But this is only what I think and belive.
Good luck with your choices. And give both kitties hugs for me.

Stella
R. P. Cathouse Maine Coons