Aye well. We had one, then found two was more fun, went back down to one, fretted and huffed and went up to three, and I still keep eyeing up strays.
I reckon if you can have a cat per room, and one piece of neutral ground, then you're probably doing okay. Having said that, I know people living in a three bedroomed house who at the height of their cat fostering/rescuing/ownership had anything from 3 to 12, and harmony (for the most part) generally reigned.
I'd say the temperament of the cat you're looking to introduce is ore important than gender. We have two boys and a girl, and they get along well. Moray is around 18 months to 2 years old and male. Malt and Barley are girls of just 8 months old and bundles of adolescent mischief. We also have two visiting neutered male cats, both around 18 months to 2 years old, and they all get along very tolerably.
I'd make sure you have a variety of places for the cats to hang out, be it cat trees, hooman beds, chairs, tables, bookshelves or whatever. Something for everyone and a few spares.
Good point Dawn, hence my thoughts about picking the temperament of the new cat to try and work as much as possible with the existing cats' personalities. For example don't get a crazy, angst ridden neurotic feral, if yours are all laid back. Or Don't get a timid, quiet cat if yours are young hooligans. (Or old hooligans, come to that).
Dawn alas didn't have much choice with her adoptee - she has a big heart and couldn't say no to very difficult circumstances. Hopefully, you'll a have a lot more say and thought to contribute to enlarging your cat family.
When we got Moray, the cat we had then (beloved Tinks) was really upset. We'd lost his best buddy, Ross. He didn't understand why this little squirt that looked like Puss from Shrek had turned up and usurped his housemate. He made his feelings known in no uncertain terms, as he came to grips with his loss. Patience and the sweet temperament of Moray won him over, but I agree, it can be difficult transitioning new cats into the equation. (Don't even get me started about two semi-feral kittens with velociraptor tendencies.....)