I'd give her as many as you think she can set on.
She'll be there for three weeks not laying and blocking a nesting box.. May as well get as many chicks as you can.
I learned that when TWO of my 5 month Cuckoo Marans went broody at the same time. I gave them each 6 eggs, but wish I'd given them a dozen each. 9 hatched but I've lost two, so down to 7 chicks.
4 was all I could talk hubby into. Not only that, she's so tiny. She just manages to keep those 4 under her lol. As it is, our coop is full and we can't build an extention or additional coop until at least next spring. I had to tell him that we would eat any cockerels that come out of this batch.... therefore I'm hoping they're all pullets lol. He's been rather stubborn on this chicken math thing. It hasn't affected him as much as it has me but he's slowly giving in. Last time I mentioned "when we get a broody" he said no they can't have eggs. Now that we have one he just rolled his eyes lol