Well, if they're inside the coop housing for most of the day (unless I misunderstood you), then I'd advise sticking with 12. However, if I did misunderstand you, and they're out in the run during the day, then you could have upwards of 30 chickens if you had enough roost space...
If you live in a year round mild climate, then you could have more than that... How many are you wanting to keep?