A broody hen should be able to cover all the eggs at one time. If she cannot cover all of them some can get pushed out from under her and cool off, even die. Then these dead eggs get pushed back under her and others are pushed out to die. You often do not get good hatches when the hen cannot cover all the eggs.
Depending on your weather when she hatches there may be another risk. The baby chicks grow very fast. If she cannot cover all the eggs she soon cannot cover all the chicks at one time and keep them warm. In warm weather this is not that big of a risk but in colder weather it can be. A hen can cover more newly hatched chicks than eggs because the chicks will crawl up under her wings and feathers as well as directly under her, but as fast as they grow the chicks soon become too big to all be able to find a warm place to sleep, let alone warm up during the day.
Eggs come in different sizes and hens come in different sizes and body shapes. I've had a hen hide a nest and bring off 18 chicks. She raised all 18 but I never did find that hidden nest. I don't know how many eggs she started with. I had another hen that had trouble covering 12 eggs so I only gave her 10 to hatch. I can't tell you how many eggs are too many for your hen but if she cannot cover them all I'd remove a few.
