How does the number of eggs affect a cookie recipe? If a chocolate chip cookie recipe calls for two eggs and I add three eggs, what will the effect be on my cookies?

Likewise if it calls for two eggs and I only put in one egg what will the effect be?
Eggs contribute to better texture, leavening and they extend shelf life. More eggs = moister (sp!) cookie. Eggs are also crucial in building structure. They are about 75% moisture, 12% protein, 10% fat and 2ish % sugar.

The white provides strength, stability and moisture.

Yolks, where all of the fat is in an egg, increase richness, tenderness and flavor.

Therefore, if you put an extra egg, you will get a chewier cookie. I do it all the time. If you put less, you will get a more crumbly cookie.