It depends on the main usage for the coop - my coop is mainly the lockup for safety at night, and for nesting. The chickens can go in and out of it all day if they want, but they also have a large fenced run. My current coop is only 5ftX6ftx7ft tall. (that's because I used a truck topper for the roof!) So, if you're going to keep chickens in that area and not let them out into a run, you can't keep too many. In my little coop, I have 7 chickens, and 3 or 4 more would fit on the roost.
I'm up early, with the chickens, and open the pop door at dawn. I would not want them to be squished together in a small space for long!