There might be a municipal by-law governing how many you can have.
We've had four. Our two oldest died last year at ages 18 and 20.5. So we were down to two till just this very morning.
This morning we got one of the ferals we've been feeding all winter in the live trap and she had four wee kittens in her nest. Yay.
Too late to prevent kittens happening at all but soon enough to socialize them up for placement as pets. I'm guessing they are 3 weeks old. A neighbour lady was out and saw us bringing them in and she will take a kitten.