It depends. I have 20 or 30 in a 16.5 gallon (I removed the bigger guys to another tub to allow the smaller siblings to catch up in growth) and another 15 in a 22 gallon. I have 6 adults (4 from breeders and two females from Walmart and PetsMart).
I'll be doing another spawn next week or the week after that so add 100-500 more to my count. Also I'll be adding another few tanks for female housing.
*The bettas in tanks together aren't adults. They're 9 weeks and 6 weeks old.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.