Two females or more females are fine together. More than two isn't necessary but it doesn't hurt either. Males born from the same litter AND brought up together to the point where you place them in their cage will do ok, though on rare occasions things have happened. Neutering male rats is the perfect solution but make sure all those little hormones are gone, and even then socialize the two neutered males gradually. Neutered males can live with the females, again, if the group a chance to get to know either other first.
Neutering male rats also keeps the rat colony at zero population control, no little ratties to find a home for.
My article Rats as Pets 
