I'll assume we're talking about "once they start laying but before they get old enough that production declines".
100 dozen = 1200. Basically 4 eggs a day, allowing for a bit of a drop in production during the winter.
You figure an excellent layer, like a production white leghorn or a good sexlink, may lay something like 280-300 eggs per year. (that you could count on. obviously a few overachievers will do better). At that rate, it would take 4-5 hens to get your hundred dozen.
A reasonably good layer, that gives you eggs 3 out of every 4 days and doesn't take TOO much of a break during the winter, would give you something on the order of 225-250 eggs per year. At that rate you would need 5-6.
A hen of a decent-laying heritage breed might give you an egg every other day and take a month or two off during the winter. That's like 130-150 eggs per year, so you would need 8-10.
A hen of a breed that doesn't lay especially well is going to put you lower than that.
Remember though that you may get some hens that are just not, individually, laying very well; also they won't start laying for 4-6+ months and after a couple years production may start to taper off (depends on how much they were laying in the first place, and management, and luck). So you need to figure that into your calculations and decide what you will do with those less productive birds.
Good luck, have fun,
Pat
