I have often heard it said that cat ownership is a lot like chocolate chip cookies--you can never have just one. While this may be true, it is important for cat owners to realize that many problems can arise from keeping several cats in a household together.
It is true that cats are social creatures, but the confined nature of our homes is very different from what social living cats experience in the wild. Feral cats often co-exist in very large, complex social groups. These groups are generally made up of related females and their offspring, as well as several unrelated males. The ability for so many cats to live in relative harmony in the wild is due to the fact that each group has a large home range and each individual has an established territory within that home range. When we bring cats into our homes, we have to realize that our house and yard basically comprise their home range; home range size may be greater if the cat spends more time outdoors than inside. In multi-cat households, each individual cat will stake out a territory within this home range. It just follows then that in a multi-cat household, each individual cat will have a fairly small territory, especially if all of the individuals are living strictly indoors.
So, how many cats are too many? This is a very difficult question to answer since this depends on the size of the home range, the number of territories available, whether any of the cats are related, and the individual personalities of the cats involved. In some cases, it only takes the addition of one, unfamiliar cat to completely disrupt a previously peaceful multi-cat household.
There are steps you can take to increase the chances of successfully introducing a new cat to your household. First, you will need to determine how your current resident cats define their territories. Do they share food dishes, water bowls, litter boxes, and resting areas? Does one cat spend all of his time in your bedroom while the others are content to sleep on the back of the sofa? Once you have figured out which territories are already in use, you can bring in the new cat and establish a separate territory for him. This territory should include his own food and water dishes as well as a litter box. Once the new cat is comfortable in his new territory, then you can begin gradual, and controlled introductions to your other cats. Reward your cats with food treats if they behave in a curious or friendly manner. Never directly punish your cats if they are unfriendly to the newcomer. Instead, allow the newcomer to escape back into the safety of its new territory and try again later. It may be the case that the new cat will never be accepted into the group. While this may be difficult for you to understand, you should try to accept the fact and go on as long as full-scale war is not erupting in your home.