I think how many you could have depends on the sex of the ones you already have. In my experience sexing them isn't as easy as some of the other cichlids, but males will have a darker spot on their operculum than the females have and another spot partway down their body. However, if they are poorly bred it wont be easy to identify. If you have two females I would suggest adding a male, if you have two males I would suggest you rehome one of the males and get either a single female or two females. But with these guys, I think a pair or maybe a trio in a 30 gallon is probably your max. But that's just my opinion. Large bodied fast moving schooling fish or armored catfish would be other options for tank mates.