I've been breeding/raising JDs for a couple years now... when given proper diet/environment... I expect Dempseys to reach 1" (from birth) in about 4 months... then grow to about 6~7" from 4~16 months (from birth)... then slowly grow from there...
You could probably have 2 breeding pairs in a 120 gal but it's likely to be a rough tank... 3 males would be risky and one would likely thrive while the others suffer... 5 females would probably work...
But in my opinion one male & tankmates... or a pair & tankmates... or 2 females & tankmates would work better...