Rule of thumb is one box per cat plus one.

I have 5 cats and 8 boxes. I'll be adding two cats this spring so I'll also be adding two more boxes. I used to think the rule of thumb was outrageous and crazy but after some peeing issues, I discovered there's a reason for it! I wouldn't need so many boxes if I was home all the time to scoop 5 times a day. The kitties almost never use the box in the basement or attic, but its there in case they need it.... and they do use it when they need it. The remainder of the boxes are in bedrooms and my office. 4 are in the office and they are the most often used!