adding eggs well after a hen begins sitting on a nest is not a good idea... it will result in a staggered hatch, meaning that most of the eggs will hatch at one time, then the hen will have to set on the remaining eggs at the risk of neglecting her already hatched chicks, or get up to care for the hatched ones and abandon the remaining eggs to die before they can hatch. With a 10 or 11 day difference I would expect her to abandon the eggs and let them die rather than neglect her chicks, but that isn't a certainty.
For best results all around I would remove the newest additions to avoid trouble at hatch time. You can always place them in an incubator and maybe (certainly not a guarantee) you can graft the chicks back to her after hatch, though again, a 10 day or so difference in chicks ages is probably going to result in the younger chicks not being able to keep up to the older ones or even be picked on or injured by the older ones. If it were my hen I would either ditch the latest eggs or incubate them with the understanding that they would have to be raised by me in a brooder.
another issue.... a hen has to be able to cover her chicks, especially if the weather is chilling off.... and not only as new hatches, she needs to be able to adequately cover them for 3 weeks or more. If you have her hatch 18+chicks then at some point there are going to be some who just won't fit under her unless she is a very large hen. So expect that you will be responsible to provide her a safe and possibly artificially warmed area for a few weeks until the chicks are feathered. They won't need brooder level heat, but need it warmer than 20 or 30 degrees at night to prevent the 'overflow chicks' from freezing..
You don't say whether the hen chose a safe spot to nest. If it is somewhere that you can provide her a safety fence or cage over the nest then let her stay where she is, if it is not a spot that can be secured then you may be forced to choose between risking the move to a safe nest and the risk of loosing the hen and future chicks to a predator. At the very least you will need to set up a spot to move her to after the first chicks hatch. Cheeping chicks are at a huge risk of attracting the attention of predators.