I'm with momo on the one litter box per cat ratio. It would offer them some comfort everytime they step into one especially if you clean up once a day or if the number of cats you have certainly seems to leave you with an "overflow" in the litter boxes at the end of the day. If their' paws start picking up soiled litter or if they start doing their business outside their litter boxes; then it is time to increase the number of litter boxes.

I used to locate the boxes at different places in my home when I had new introductions but as time goes by, they will all learn to share the litter box in a common area to do their business.

wooz....