I currently have 30 chickens (10 bantams, 20 standard).
Coop is 80 sq ft = 2.6 sq ft per chicken
Enclosed run is 460 sq ft = 15.3 sq ft per chicken
The total for both is 17.9 sq ft per chicken.
The two are connected - they have free range to go in & out of the coop, where they have nest boxes and roosts. They don't get free reign of the rest of the backyard but I give the the lawn clippings as well as daily kitchen scraps.
I feel like I am at max capacity for happy, healthy chickens.