I guess the question should be asked, what else is in your tank? And how much cleaning do you want to do. You could "technically" house 24 3 in tetras in there but that would be INSANE! and this is assuming that you have no gravel or decor and you had the filtration system of sea world. Why not try a school of 8-10 and see how that looks if you are into species only tanks.
If you have other fish in the tank, then you might have to decrease this number again.