Unless you consider a larger tank like a 55 minimum, I would stay away from Mbunas.
However, even with a 55, more than 3 or 4 males with 2 females would be the absolute maximum.
Those creatures use up a lot of space while creating there territory and some of them will defend it pretty fiercely
"Controlled overstocking,” as Oscar_Wild said, will be reached with 2 males and 2 females in a 20. (IMO even with 1 male and 2 females).
My advise; Look for a 55 or bigger if you want Malawi cichlids.
If you stay with the 20 gal. read upon some "Shellies" from Lake Tanganyika. They are just as colorful (if not more), and display some interesting behavior too.
Jimmy