That depends on a lot of factors, but the standard is 1 square foot for small to avg and 2 square foot for big. I have 12 in a 4'X4' coop, it's small for the standard. My coop gets along well, has a ladder roost that fits everyone with extra room, feed/water is mounted flat to the wall to save space, and they free range. In fact, the only time they are all in there at once is to sleep. You need to look at the time they are spending in there, if its full time than you need 4 square feet at least per bird and be prepared for boredom problems. How much space is taken up by your feed, water, and roost. How does your group get along. Like I said, depends on many factors.
Edited because I hit send instead of spell check.LOL