How many cats is too many cats?

[yiskasha]

I have 3 cats. I adopted them from a rescue centre when they were kittens, all from the same litter. They were brought in with their mum who was lovely also. They're 3 now and get on really well.
I've seen a kitten who has just been brought into a rescue centre near me and for some reason I can't get him out of my mind.
How many cats is too many cats? I could afford another one, but is it fair to my cats to bring another one into their home? I've never had to do an introduction before so I suppose that's what's making me worry most.
I have 2 boys and a girl at the moment.

Well I can assure you that my house doesn't smell. (It has been independently verified) :)

I think your house only smells of cats if they are indoor and you have cat litter trays.

I’m absolutely not saying this applies to everyone! Just in my personal l experience, it does in the case of my family member. Unfortunately

I have 9. Trying to convince DH that 10 would be a nicer number . Fortunately they all get on with the exception of my psychopathic Siamese who hates everyone except one of my male cats who she never leaves alone!