Size In Your Opinion

I am planning on building a 3' x 6' cage...not cubes, feet. I don't use cubes because of my evil cats. Back to the subject though....the cage would be 18 sq feet. How many females would that be a nice size for? I presently have 2 and am hoping to adopt more but I don't want them too cramped. I was wondering because I was planning on adopting one but lots of the ones I see would like to be adopted with a friend. Anyway, thanks for your input.