Any more then one pair of convicts will not work in a 20 gallon tank, and unless the tank is a 20 gallon LONG I would not recommend that either. It is very common for people to think that convicts can be kept in tiny tanks, but this is not the case. Males should reach 6" long and they become extremely aggressive when breeding.
If a 20 gallon is the largest tank you can keep then I HIGHLY suggest finding more suitable fish. Dwarf riverines are perfect. The question becomes do you just want to keep fish, or do you want to do it properly? I myself would not house a pair of convicts in anything less then a 30 gallon, 36" tank.