I have a glass pen that was originally going to be a coffee table terrarium. I partitioned it into two sections and am currently running two groups 1.2 rats (on each side) the partitions work out to around the size of a 33 gal. Everybody comfortable, everybody producing. Ventilation is no more of an issue then in a cage. They stink either way..
So one adult rat per sq. foot is pretty good. If you are keeping it for juvi rats you can definitely go more as they tend to pile themselves up in a corner anyhow. I've had 20 (50-65 gr. rat juvis in a breeder cage) and nobody was tripping over anyone else.
Jim