I typically give a hen 12 eggs the size she lays. She needs to be able to cover them well. I like to leave enough room for a couple of extra eggs in case some new ones show up, which can happen as I let them brood with the flock. I had one that had trouble covering 12 so I reduced it to 10. The most I've ever purposely given a hen was 15 eggs. She was a fat bottomed girl and had no problems.
I've had a hen hide a nest and bring off 18 chicks. I never found her nest so I have no idea how many eggs she had but she obviously could cover them all.
When a hen will be raising her chicks in cold weather I reduce the number of eggs some. I want her to be able to keep all of the chicks warm in cold weather after they have grown for a few weeks. In warm weather that is not a concern of mine.
Each hen is an individual with her own body build. Each situation is different. I try to go more by what I see than following specific numbers.
Good luck!