My rule of thumb is one litter box per cat and at least one extra. In my house we have three cats and seven litter boxes. I know that's a little bit extreme but litter is cheap and carpet is expensive. Once a cat looses its natural inhibition against house soiling it is very difficult to retrain them to use the litter box.  Buy a carpet cleaner- check out the Bissell Spot Bot on our "links we love" page if you need an excellent small carpet cleaner.

Irate  cat owners may be surprised to learn that very few cats eliminate outside their litter box because they are mad or trying to express some other complex emotion to the owner. It usually just means the litter box is dirty.  Eliminating outside of the box actually serves a function for cats, especially in a multiple household. It is a way of marking territory. 

When we get a case here that a cat is eliminating outside the box we  check a urine sample just to make sure the cat is urinating normal urine. Some conditions like urinary tract infections and bladder stones make the urinating uncomfortable for the cat. The cat then associates that pain with the litter box and starts looking for other places to urinate. We are happy to find these cats have something treatable as our success with medical problems is actually much higher than with pure behavior problems. House soiling can be a life-threatening problem in some households so we take these problems very seriously.

if the urine is normal then our job is to make the litter box as attractive as possible. Sometimes will do a litter box smorgasbord- a collection of litter boxes filled with different types of litter, covered and uncovered, scented and unscented. I will do up to 12 at once. Once the cat has voted we provide multiple litter boxes and throw the litter out daily. If that solves the problem we are lucky. Some cats require medications, pheromone sprays or even behavior modification therapy. It is far easier just to have a lot of litter boxes and always get the cat checked out anytime they eliminate outside the litter box!