Exactly 157. (Just kidding!) Actually, how many eggs a chicken lays a year depends on a lot of things, but especially on the breed of your chicken and her age. For instance, young Leghorns may lay 300 eggs per year. Young Cochins may lay only 100 or so. Sumatras may lay 50 or fewer.
By breed, you can see a general idea of how many eggs chickens may lay per week here on My Pet Chicken's chicken breed list. But keep in mind that as the birds get older, they will lay fewer eggs. And some breeds are not great winter layers--- cold hardiness and winter laying are not the same thing ---so they won't be laying 52 weeks a year. Other things that figure into how many eggs your flock will lay include how well they're fed, how stressed or relaxed they are, and how healthy they are.
Chickens need a good quality feed that is nutritionally balanced and has plenty of calcium for strong egg shells. And they need to be healthy to produce eggs, too: if they're suffering even from an infestation of mites or lice, this can interfere with laying. Being stressed out can reduce laying, too! So being harassed by a barking dog---or even being harassed by an overly-aggressive rooster---can decrease the eggs you get. Even a loud thunderstorm might disturb them and throw off laying. And certainly being uncomfortable in situations where they're overcrowded, hot, cold, and so on can stress your birds out.
So if your concern is to produce the most eggs possible with the fewest birds possible, get a breed that can lay high numbers of eggs. Then provide your birds excellent, roomy, clean shelter, as well as excellent feed and care. Finally, don't forget that eggs are tastier---and healthier---when your birds can forage and engage in their instinctual behaviors. If you want to be buried in fresh eggs, you may as well be buried in the healthiest and most delicious---and humane--eggs as possible!