If you leave her to pile up her eggs in the box she'll decide when she's ready to sit. I did this with 3 of my first timers, they sat on 12 eggs every time. I think that was the number they felt comfortable with.
I gave my broody 6 turkey eggs and by morning she had moved one of them about 2 feet away from her and was sitting on 5. I assumed the 6 eggs were too much and she was comfy with the 5. She's still on them now.