Hmmm...I havent' used a closet for a litter space in a while. I am going to have a large walk in closet in our new house. That would be perfect. But I do like to have a window for air circulation....we'll see!
I have 8 cats. Our old house was smaller, about 1000sf including a partially finished basement. There we had 6 boxes...3 in the 2nd bedroom/office, a carpeted one in the living room, and 2 in the basement. We left the basement door open.
Our "new" (very old) house is more than double the size of our old one, so I found I needed to add more boxes for the new spread out territory. 2 stories was a new experience for the cats too...although they did have the access to the basement at the old place.
When my cats still free fed on their dry upstairs, I had 6 boxes upstairs and one downstairs in the bathroom. That worked out fine. Now they don't eat dry food, and they do all their dining in the kitchen downstairs - so one box downstairs wasn't nearly enough. They seem to do most of their pottying right before and after they eat.
NOW, we have a total of 11 boxes. 3 upstairs in a large closet in the guest bedroom, 3 in the spare/cat room upstairs, 2 in the walk-in closet (with window ) of the spare/cat room, one in the bathroom downstairs, and 2 more in the same bathroom, but in the cupboard under the sink. We leave the cupboard door cracked just about an inch and they seem to enjoy wiggling through the opening...so it works.
The 3 downstairs get used a lot so I scoop several times a day. I usually only scoop upstairs once in the morning and once before I go to bed. Those don't get used much.
Overall I'm real happy with the layout here. The closets are big so there's room for the boxes and still room to spare for storage. I've managed to avoid having any sand in the main living areas...but they do still have plenty of places to "go"...so it works out well.