Sorry - but there is no simple answer. You'll get different answers from different people.
Most here on BYC suggest minimums of 4 sq. ft. of indoor space and 10 sq. ft. of outdoor space per bird. But folks in year round warm areas do fine with smaller indoor spaces than that recommendation, while folks up north who get deep snows want MORE than the recommendation. Then there's bird size - One person may have 10 lb. JGs or Brahmas, while another has tiny 1.5 lb sebrights, and breed activity level (some breeds are very calm, handle limited space better than active breeds). Some are contained within a run all day, while others spend a few hours per day outside the run - so the run is a bit flexible too.
So there's more to it than just a simple answer.