Ok, so is it going to be 3 sided? Will they free range outside? Is this in a fenced run? If so how big is the run? If not a run and free ranging, how much space do they have to range? You can fit quite a few birds in that if they spend a lot of time outdoors. If they are only roosting and nesting in the coop then you can do 18+ birds. You need 12 inches of roosting space per full sized bird, and 1 nest box for every 3-4 hens. So basically the number of birds you can keep in there depends on if they are kept inside all the time or if they free range, and how much they free range. There is really no definitive answer. You stated the front is open, that leads me to believe they will be free ranging at will. Hope this helps. Bear in mind that if they are confined, and you have too many you will have problems with pecking, and behavior.