In a stable population, as long as each cat has a resting place, a hiding place, access to food and water and litter box facilities, I think the big limitations are your time and square footage. New kittens can always be integrated, with a little time and luck.
However. (There’s always a ‘however’).
Cats, just like people, can be slow to accept change, especially when it’s pushed on them by someone else.
Imagine, you’re a comfortable cat. Middle aged, maybe a few ounces over ideal weight. You’ve got the world figured out. You spend much of the night on the foot of the bed, cuddled up in the folds of a down comforter. Early in the morning, you stretch, yawn and begin the arduous process of reminding those tardy, sleeping humans that breakfast is due. Now.
Snack, litter box trip. Maybe a few sips of water. Then it’s time for the mid-morning nap. Repeat three times and the day is fully experienced.
Maybe there’s a lovely window with a view of a garden or a bird feeder. Maybe a bug gets inside and you spend hours playing with it before finally eating it. If you’re lucky, your humans will have a play time in the evening. Laser tag sometimes. The odd catnip toy. Or those really fun little bouncy things.
Oh, and a scratching post. The cardboard one with catnip inside it. Jammed in a corner near lots of traffic. The perfect place to let the world know you are the reigning monarch.
Then the traitors bring home a kitten.
Isn’t he cute, they say. He’s so friendly and playful, they coo. They can’t see what the cat can see.
The little turd hides around corners and jumps on your head. He waits outside the litter box and gnaws on your ear when you are trying to make a graceful exit. He eats your food. He sleeps in your spot. And if you get rattled, or cross, or even think about telling the little beast how to act what happens? You get scolded and locked in the bathroom.
There is some promising new research out about how to reduce stress between cats in a household.
A new pheromone product, based on the scent from the mother cat’s mammary glands, has helped to reduce aggressive behaviour among cats in the first study on the product.
Feliway Friends is the name of the product, and your veterinarian should be able to get it for you.