Roughly 2.5 wks ago, I took a female mallard in to a wildlife rehab center who had been hit by a car, and subsequently expired. She had made a nest, and I was told that no other mallard would incubate the nest. The nest is now empty and there is another mallard that as of yesterday arrived at the pond with 18 ducklings. Is it possible that she had 18 eggs in her clutch, or could some of these be from the clutch of the deceased mallard?

Mallards typically lay between 8 and 12 eggs per nest. However, clutches of up to 25 eggs in one nest have been documented in Minnesota. So the ducklings that you saw with the hen could have all been her offspring. Another possibility (though unlikely) was that both hens had been laying eggs in the same nest (this happens occasionally). In which case, some of the ducklings could be from the hen that was struck by the car.