One in our laundry room and one in the downstairs bathroom. Our litter boxes are actually massive black containers used for mixing drywall, and they only ever use the one in the laundry room!
Two in a converted closet we opened up and use it for that purpose, and one in the sunroom. Both of these areas are designated "cat" rooms and they're designed for them, and not people.
I built this house and the plans called for a "Mudroom" at the back door. I added a window and turned it into my den. It is small at about 8X10 feet. I built a bench into the wall and have my ham radio gear on it with shelves above. I put his litterbox in the corner. He likes to lay in the window in there when it is open. His hobby is bird watching.
Mine is in the 2nd bathroom which is not used for shower. This makes the bathroom dry most of the time so it’s easier to clean/sweep away stray litter. Not a fan of putting litter box anywhere else since that is a ‘toilet’.
I have three litter boxes in the bathroom, an extra in the hallway, and covered one in the living room in Menzies' territory, because she likes her privacy. I used to have 6 (for 5 cats), but found they all (except Menzies) preferred to share the same three litter boxes, so all I was doing was making more work for myself.
In a nook that’s kinda behind the door in my office. He likes his privacy, he has a top entry box and gets annoyed if you look at him while he’s peeing so we made him his own little litter box hideaway. Fine by me, then I don’t have to look at it!
I really wish it could live in the bathroom since I have an ADA accessible apartment with an enormous bathroom, but when we tried that he decided that batting little clumps of litter around the tile floor was a super fun game and started kicking stuff out on purpose so he could play with it. Cats are so frustrating sometimes! I buy you toys and you want to play with poop sand instead!