How many gerbils can I have in a average size glass aquarium?

I got my niece 2 gerbils for christmas but she recently pulled the tail off one and decided she didn’t want them anymore. I already have 3 in a glass aquarium from the same family as hers and the same sex, but I’m not sure if it will be a little cramped for them. I can’t use my niece’s cage as they make to much mess when they dig and I cannot afford another aquarium.