I think Katiemae wins for most cats per square footage!
We have a one bedroom apartment and two cats. Our friend who previously lived in our apartment lived there with three cats (at one point 10 cats: her 3 boys, and a mama cat and 6 kittens she was fostering, but they were largely confined to the bedroom and not terribly mobile). I think our apartment could support another cat; my boyfriend disagrees, so until I can get him to budge, I'm stuck with two.
Bear in mind that cats view territory as being both horizontal and vertical. If you have places for them to climb, that increases the amount of space they can claim as their own territory. Two (or more) cats can be quite happy in what seems like a small space, so long as there are multiple levels for them to explore.