you could keep a good three or four in 55G but might have to upgrade to like a 75G or 80G eventually but make sure for how many you got have each have its own territory..but also depends on what species but im assuming red bellies..
ive had red bellies forever and i currently own jus one now its about 3-4 inch and i house it with a silver arowanna 3 plecos and two cons male and female with no problems what so ever. but imma tell u this NOTHING can live with piranhas but piranhas