According to the American Association of Feline Practitioners in partnership with International Cat Care (ISFM) the general rule of thumb is to have one litter box for each cat, plus one extra box in multiple locations around your home.
Socially affiliated cats, which are two or more cats that are familiar to each other,share a territory, and exhibit behaviors such as grooming, playing, or resting together, may be more willing to share litter boxes. Because more than one social group may occur in a home, providing adequate resources for each group is important to decrease the chance of adverse behaviors.
For more information from the AAFP and ICC on cats and house-soiling, see their brochure here, sponsored by Ceva.
