May I interject a thought here........
A lot of people talk about the SIZE of the box. (i.e. I have only 2 boxes but they are larger than regular litter boxes). I really don't think this matters unless 2 cats would be able and willing to use it at the same time. I always took the "rule" of at least one box per cat to mean that IF all cats had to potty at the same time, they'd have a place to go. See my point?