>I've read that the general rule of thumb is to have one box
>per cat, plus one extra.
I've read the same rule in multiple sources, and I think it's a good rule of thumb. At the VERY LEAST, I think there should be 1 box per cat, in case of territorial issues, so each cat can "claim" a box, if need be. We have 3 cats and 3 boxes, only because we don't have anywhere else to put a 4th box.