Conventional wisdom on this site uses 4 square feet inside the coop and 10 square feet outside run space as the minimum per bird. Some variation is possible depending on the size of the birds to be housed. The space described should easily hold 10-12.
Available space should include only "free" floor space not otherwise occupied by nest boxes, feeders, etc.
Your climate will also affect the numbers as chickens do not much like winter weather and tend to stay inside much of the time. This adds to flock pressures and can result in aggressive behaviors between flock members.