African or New World ? There's a HUGE difference in stocking between the two.
Both Continents have dwarf species so if you have a proper 50 gallon and not a 50 gallon high tank then I would say maybe 4 that get to 6in each from either.
Maybe like 8 if they are dwarfs and stay between 2 - 4 inch's
Just make sure they are all aggressive enough to be together, even though they are Cichlids they will get beat up by more aggressive one's.
My favorite New World that would fit nicely in a 50 is a Salvini Cichlid. They show good colors and when mating turn into three beautifull colors. This also earned them the nick name Tri Colored Cichlid. A breeding pair would be nice in a 50 gallon.
If going african I would go with electric yellows, they are a dwarf species and only grow about 4 inch's. They look good when kept together, in a 50 you could have about 10 and should have at least that so they have plenty of fish to chase and no one fish gets picked on.
Remember that both groups require vastly different water conditions and tanks decor/set-ups so do your research before you buy and set up the tank.
African cichlids would do nicely in there. Remember though, they
grow (although not as extreeme as an Oscar or Blood Parrot).
I wouldn't put more than 5, because when they grow, you might have
too small of a tank.
What kind of Cichlid set up do you want? There are over 1200 species in that family, many with very different needs.
Did you want a community tank or only one fish? You cannot really give an exact number answer here without knowing what kind of look your going for. What are your water parameters also? That will dictate whether your best off with American or African Cichlids.