Wait.. Why'd we have a question about how many betta fry should be in a tank and have a reply to what they eat? o.o.... *shrugs* Anyway! From what I recall, the minimum size tank you should have for betta fry is 20 gallons- You want a good sized tank so that it can heat properly, have fantastic water quality, and let them have room to grow out and be healthy little guys. I cant' really say how many per gallon as I've never taken care of fry before... but to me it seems like you would want to have a pretty good sized tank regardless... You should look at the stickies in the breeding section and maybe ask questions there and/or look through threads of other peoples baby betta's. Usually if you're not UBER diligent with the care and responsibility of them and don't have the right conditions, you will have dead fry on your hands.