The above is true but also...........................hatchiblity holds up to day 7. After that the chances of the egg hatching is greatly reduced. by day 14 hatchibility will be about 50%. It's also important to store the eggs with the pointy end facing down.
Keep your setting eggs in a dark and quite place at 55 degrees and 65% humidity. Turn the eggs 1, 3, 5, 7, or 9 times daily. Discard all imperfect eggs as soon as they are laid by having them for breakfast. An egg 21 days old while still sellable at Whole Foods, is usually a waste of time if you want to hatch it.
The truth of Emma14's post is that 15 eggs have long been the number of eggs supposedly in a setting of hen eggs.