Mike,

The rule of thumb is one box for each cat. But I've also had 2 cats share one litterbox. It all depends on the individual cats and how they react to sharing.

The Littermate came out in late 1995. My Dad wanted one when he saw it on the informercials (long before they came out in stores and were priced at $199!!) I was not that impressed with it because you can't put alot of litter in it or the rake will get jammed. If your cat likes to dig, it can be a problem.

The Littermate has come a long way. So my opinion is based on the very first one which I guess isn't much help.

So, which new kitty are you getting? Good luck with your new addition.