I think it's all about how well the cats get along. Smokey and Farrah, the cats I had growing up, didn't get along well after Farrah became an adult (Smokey would generally hiss if Farrah got too close, except at meals and on very cold nights, when they managed to peacefully share my parents' bed). I think my parents' 1700-square-foot house was just about the right size for two non-friendly cats.

(We probably could have gone to three if we really wanted, but certainly no more than that.)
If Misty and Stormy got along like Smokey and Farrah did then my 780-sq-ft apartment would be way too small for them both, let alone for more.
On the other hand, given that Misty and Stormy
do get along I think my apartment is plenty big enough for a third cat... provided he got along with Misty and Stormy as well as they get along with each other. I think more than three would be a stretch no matter how much they liked each other.
One thing you might want to think about it maybe waiting for a bit. 5 months old is still a kitten and adult cats sometimes get along better with kittens than with other adults. It might be worth waiting until you're sure your older cat will still like the younger as an adult before deciding to get yourself into a third cat.