I dont know the minimum, but a good recommendation is 55g or more... I remember a fellow in another forum who kept them (think it was a 75g or more) and noticed a change in their behavior when the group size exceeded about a dozen. I think he ended up with somewhere around 16 or more of them and said they had fantastic color and activity that was not apparent before when he had fewer of them and they were not the dominant fish in the tank. Just some food for thought. There are, however, many other tetras more suited for 20g 
