you might get away with 3-5 julies if you pick a small species and only get 1 male. if you get multiple males get rid of all but one of them and try to get extra females to spread the aggression. the general rule with africans is 1 male for every 3 or 4 females, males of the same species kill each other in a tank that size. honestly if you want a good number of fish the shell dwellers would be a better choice since they only get about 5 cm long, you could set up a nice colony of small shell dwellers in a 20 gallon.