Hens and eggs come in all different sizes so it is hard to come up with a hard and fast number. Some bantam hens can only cover 4 full sized eggs, while others can cover more. As long as you can't see any, then you remove one or two, you should be OK. But all bantan eggs are not the same size.
For my full-sized hens, I've seen a hen incubate 18 eggs and do OK, but that was a hidden nest. When I set eggs under a broody, I usually set 12 eggs of the size she laid. Top me, that is a nice hatch and I have some extra room in case another hen lays an egg with her. What you want to avoid is the nest being so full she cannot cover them all. An egg can cool off and die, then get moved back under her while another egg gets pushed out to cool off and die.