One of your large sized broodies can hold up to about 8 eggs (depending if they're giant ones) Sometimes even more. You don't want any hanging out, so you can kind of estimate how many she can hold. I always do 6 or 8 eggs under a standard large hen.
I read somewhere to estimate on the short side. She rotates eggs so at some point they are all on the outside and if they are not warm enough on the outside you can lose a lot of eggs. After they hatch you can add chicks. Our latest broody, Big Buff, set eight eggs and hatched six. You don't get to pick which ones hatch. We will add eight more chicks now that the hatch is completed. We get to choose what chicks we add to the brood.