My vet recommends one litter box per cat plus one extra. I have 4 cats and 8 litter boxes!!! Sounds like a lot doesn't it? It is a lot of cat poop!
I have all of my litter boxes in my basement which I do not use for much of anything else. I have cat doors on my bedroom door, basement door and computer room door. My cats' food is in my computer room, hence the cat door so the dog cannot get in there and eat their food when she is unsupervised. I truly hope you can get your kitties settled inside. I am very sensitive to this issue because I have seen what can happen to cats that are left in and out. Keeping them inside will also lengthen their life and quality of life. Good luck!