Not all hens are the same size and not all eggs are the same size. For a tiny bantam, four regular sized eggs may be too many. For a full sized hen with regular sized eggs, I've seen them cover 15 and they might have been able to go with even a few more. She needs to be able to cover them all. I'd suggest you consider no more than 12 for a broody if the eggs are the right size for her. That gives you a little flexibility. You can go with less if you wish, but I'd suggest at least four if she can cover that many. You never know how many will actually hatch. With a minimum of four fertile eggs and a broody, you should get a couple at least.
I'm not sure what you mean about how many so the chicks will stay warm. If you let the broody raise them, she will keep them warm, whether it is one or a dozen. If you raise them in a brooder, then providing the heat is up to you.