The counter is best.
Your hatch rate is dependent on fertility and the hens' skill at sitting on the eggs. If both are good, you can easily hatch 100% of the eggs.
OK- you'll only be able to give a hen eggs to sit on, if she has gone broody. If she is not broody, she will not sit on the eggs long enough for the eggs to develop and hatch.
No, you will not have to provide heat--hens can hatch eggs all by their lonesome, if they are so inclined. There is...