Some depends on the size of the hen and the size of the eggs. Some depends on the size and shape of the nest. I normally give a hen 12 eggs the size she lays but I had one that could only cover ten well. As a kid I saw a hen with 18 chicks that she hatched out of a hidden nest. I never found that nest so I don’t know how many eggs she had to start with. I successfully gave one 15 eggs one time. There is no set number. It just depends on how many she can cover, but you may be amazed at how well she can flatten out.
I don’t know where you are or what the weather will be like when they hatch. In warm weather it doesn’t matter much but the chicks grow pretty fast. With too many she won’t be able to cover the chicks in a couple of weeks. Here in the heat of summer it’s not a problem, even with nights getting down in the 60’s. But if it is going to be cold in a month you might limit the number because of that.