Your going to get a million different answers to that question. But the general rule of thumb is 1inch of fish per gallon but there are a lot of variables to consider like filtration. I will tell you the bigger the tank the more stable your water parameters "can be" for example ammonia will spike faster in a 30 then it will in a 120. basically you have a larger "margin for error" sort of.
Long live the king
"Stand for something of fall for anything"
actually the *1 inch of fish per gallon* rule does not exist, its a myth... thats saying you can put a 12 inch oscar in a 12 gallon ( eclipes ) so dont follow that rule, but in the other side of the divider you could do a pair of firemouth, would look sweet IMO