Will they be in there their whole lives? They do get kind of big....I would only put in 3 of them. (that's if that is the tank they will live in permenatly) If not, and you are using it as a grow out tank, and moving them into a bigger tank, then I would put 6 of them, and as soon as they get to be about 5 inches, I would move them into the bigger tank.
Jen