The first thing you need to know in figuring out how many birds you are going to keep is whether there are any regulations for your location that limit the number of birds per household. After that, each household needs to figure out approximately how many eggs per week they consume in an average week (remember to include those used for baking, in other foods, etc - not just those eaten as plain eggs).
A good rule of thumb to follow that can help keep you out of trouble while learning the ropes of chicken keeping is to allow 4 square feet per bird inside the coop and 10 square feet of run space per bird. Remember that space occupied by feed/water dispensers is not space that can be occupied by a bird and do not include nest boxes in the space calculations either.
Keep in mind that different breeds have different average rates of production - so it isn't just about needing x dozen eggs a week so we get 10 (or however many) birds - you'll need to balance your number of birds with the average production rate for the breeds you choose -- afterall, a flock of 6 high production birds would give you more eggs in a week than a flock of 10 low producing breeds.
I have a flock of 16 active layers, 1 useless chicken and 5 8 week old pullets who are this year's "fresheners" for the laying flock. Out of those 16 birds, I get right around 8 dozen eggs per week during peak production periods.
