Chickens and eggs both come in different sizes. A chicken will try to hatch any sized egg, whether hers or another chicken’s. It’s all some bantams can do to cover four full-sized eggs. Some full-sized hens can cover an unbelievable number of bantam eggs. I’m not familiar with Dutch bantams, how big they are or what sized eggs they lay. I don’t know that you are talking about eggs the size she normally lays or another sized egg.
A hen needs to be able to cover all the eggs she has to hatch. If she cannot cover all of them some will cool off a bit when they are not under her, maybe to the point of killing the embryo. Then that egg gets pushed back under her when she turns them and another egg gets pushed out. You often don’t have good hatch rates if the hen has too many eggs.
Weather can play a part. In the heat of summer having an egg on the outskirts isn’t as bad as in the cooler weather (although it is still not good), but the main thing here is that the chicks grow pretty fast once hatched. In cold weather the hen needs to be able to cover all the chicks, especially at night, but if she hatches too many she may not be able to do that after a week or two. In hot weather that’s not that big of a deal. I’m a little more conservative in winter than summer when setting eggs.
Another question is if the hen is hatching with the flock where other hens have access to her nest. Sometimes other hens will lay in a broody’s nest or the broody will actually collect eggs from other nests and add them to her own. Yes, they really can do this. It’s pretty rare and I haven’t actually seen them do this but I’ve seen the results. Still, the biggest danger is that another hen will lay eggs in her nest. So you need to mark the eggs you want her to hatch and check under her every day to remove any that don’t belong. And you need to leave a bit of room for an extra egg or two if she is hatching with the flock.
I’ve seen a hen hide a nest and come out with 18 chicks. I never did find that nest so I don’t know how many eggs she started with but they were the size she normally lays. All our hens laid about the same size eggs. I’ve had hens struggle to cover 10 eggs of the size she normally lays. Most of the time I give a hen 12 eggs to start with and it usually works. I’ve gone as high as 15 eggs with some hens. I can’t give you a hard and fast number that will work for that hen and your eggs. You’ll have to use your judgment. Normally they do spread out more than you’d think so just try to not overcrowd her but be a bit brave.
Good luck!