I would not push it either. In another 4 weeks in North Carolina you will have some pretty cold nights. The chicks will not be fully feathered out before they have to survive out from under Mama. I do believe, based on experience, that they can keep a lot of chicks warm under normal conditions, even more chicks than they can fully cover, but I think 15 is asking too much in late fall or winter in your climate. In the middle of the summer, I'd easily go with more than 15, and I have successfully done that in early fall with 15. But I would not try that many now, even with the little Serama chicks. It is a little too late in the season.