Hens come in different sizes. Eggs come in different sizes. A bantam may have trouble covering 4 regular-sized hens' eggs. A full-sized hen can cover a huge number of bantam eggs. I typically give a hen 12 eggs of the size she normally lays and it usually works out.
I had a hen hide a nest and bring off 18 chicks. She laid those eggs so she was able to manage that many of the size she laid. I once gave a hen 12 but she was having trouble covering all of them so I took two away. Some people like hard and fast numbers regardless of details but I find each situation is unique and try to react to what I see.
I never set extra eggs under a broody or in the incubator counting on some to not hatch. You have to be OK if they all hatch. I have had 100% hatches, I've had some a lot worse. You do not count your chicks before they hatch, either positively or negatively.
I've had great success with a hen that had never gone broody before. I've had failures with hens that were successful broodies before. I don't pay any attention to that first-time-broody stuff, but if a hen messes up on her first hatch I never give her a second chance.