WOW, 15 litter boxes!! I wonder if it matters how many per space? Like in my laundry room, I have 2, then the guest room I have another. Would I need one in every room? That's not a pleasant thought...although neither is cat pee everywhere . Out in the cottage I have one by the front door and 2 in the rear. They use the big, long, open one by the front door more than the others. There is an open box and a covered one in the rear. That is something else I was experimenting with...whether they preferred covered or open litter boxes.