... I once grew around 40 fry in a 15 gallon but I had the feeling that was very much on the edge in terms of water quality. Besides, the fish grew really slow and I traded them when they were sexable but not yet at a good size for selling.
"The solution to pollution is dilution."
... I think the most difficult is to have the discipline of not having too many species. I ran into trouble because of having too many species and they all had fry. The reasonable solution would be to have less species but that's more difficult than it sounds!