Yeah, i mean thats exactly what i said to the store owner when i got mine as well and his thought was that as long as the tank is well established you could probably keep 10-20 in a 60G easily...
I mean at the store he had easily 50 of them in one very small tank (maybe 12" cube), but not sure how well that would work out in the long run...
So far mine have done pretty well considering i havent really fed the tank much at all, so if were to strictly have a guess, i would say maybe 4 in a 20G depending how densely planted the tank is???