I have 4
I currently have 4 all of them female, well three are female and the fourth is to young yet to know for sure.
Not sure about putting them in a 20. I have mine in a 125 with Afra cobues, Yellow tail aceis, yellow labs and petricolas.
The Astatotilapia Latifasciata are the largest fish in the tank but they are very peaceful. The afras chase them and most of the aggression, if you want to call it that, ( mild chasing) expressed by the Latifasciata are towards there own species.