When Delilah was an only kitty, we had just one box and never had a problem. She and Blue shared a single box for awhile too, until Blue decided for about a week that she preferred to go behind our entertainment center
- added a second box. (Blue uses both boxes - Delilah uses only the one she's always used.)
Now we have Midget, and she's been using box #2 exclusively. I'm hoping two boxes will be enough for the three of them, but I could add one more box at some point if I had to. 3 boxes is my limit though - I don't have anyplace to put a fourth.