Here's a 101 on chicken reproduction.....
Hens will lay an egg basically every day, usually taking off 1-2 days a week, depending on breed.
If a rooster mates a hen, she stores the sperm inside. Her body fertilizes each egg from that mating for about 2 weeks, give or take.
In birds that are naturally broody, a hen will lay an egg a day until she has a clutch of about a dozen eggs. This will vary a lot depending on the bird, a dozen is a rough average.
The fertile eggs are in a kind of stasis at this point. they don't start to develop chicks until she sets on them.
At some point around there, her hormones will trigger the instinct to brood or set on those eggs. Once she sets and her body heats them up to around 100 degrees for a few days, the blood vessels and chick starts to develop. She sets on them for 21 days and then chicks hatch, usually all over the course of 1-2 days.
Momma waits a day or so after the first chick hatches, to give everyone time to hatch. Some point on the second or third day she'll leave the nest and any eggs that haven't hatched yet, to show the chicks food and water.
In a lot of hatchery breeds, a hen won't go broody no matter how many eggs build up in a clutch. It's been bred out of them and just won't happen.