Although she could easily cover 6-8 eggs judging by the pic you have to keep in mind that with cold weather hatches the hen will be warming a lot of little bodies, and they won't be little for long. I tend to reduce hatch sizes by about 25% in cold temps to keep plenty of room under the hen for chick warming. I would aim for 4-6 chicks for a medium hen in winter.
X2. Having a heated coop I did not consider winter conditions.