Depends what is most important to you.. Getting as many chicks as possible from these eggs or having a hen that incubates her own eggs. If you leave the eggs there, she might go broody within a few days. But the chance she doesn't is probably higher. If you remove the eggs, she is not going to go broody - before she's laid another clutch, at least. But it increases your chances of getting chicks from these eggs and not having to wait for the next clutch.
I think 12 is the highest number I've seen a hen able to cover, so I'd definitely remove them when you reach number 12, if she hasn't gone broody by then. Fertility is highest if the eggs are fresh though, and people usually recommend that the eggs should be less than 10 days old when you put them in the incubator. Your call. But I think I'd leave them till you have 12 eggs and then pull them for incubation. Perhaps the oldest ones won't hatch, but then you've given her a reasonable chance..