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There are many good coop plans that are easy to build, or you can make up your own plan. I'd check out the coop page and see if there are any coops that catch your interest: Chicken Coop Plans Or, you could post some questions in the Coops thread: Coop & Run - Design, Construction, & Maintenance
As for how many chickens you should get, that will depend on how many eggs you want and how large your coop will be. Keep in mind that each standard sized chicken should have 3-4 square feet of indoor coop space. You'll also need one nest box in the coop for roughly every four hens. During the summer, many birds will lay 5-7 eggs a week each, especially if they're one of the more productive breeds/hybrids (Sex-links, leghorns, Australorps, etc.). In the winter, egg production will slow down or even stop unless you provide supplemental lighting (14 + hours of light a day is best). Also, most birds will molt for 1-3 months each year, during which time they don't lay.
As long as you can prevent overcrowding and take care of them well, there really is no such thing as too many chickens. I would, however, caution against getting only 2-3 hens, as chickens are social creatures. If you have two hens and one dies, you'll have one lonely bird remaining. If you have three hens and one dies, you'll be left with a pair that is at risk of eventually resulting in a single hen. I'd recommend at least four hens.
Edited by BantamLover21 - 11/20/15 at 2:18pm