I'm new here but I understand that it's widely accepted that each hen needs around 10 square feet in the run (slightly less for bantams), so that would bring you up to 120 sqft for twelve and with the possibility of chicks, you might want to bump that up a little. This is of course if the chickens are not going to be free-ranged most of the time. If they are going to be out most of the day you can cut that back a little. They need 4 sqft per chicken inside the coop, and again, less for bantams.