How much chicken to feed 100 people?

Figure roughly 1/2 pound per person and add 25% (that's 12 ounces per person, so 75 pounds).

It depends too on what type of chicken, the cut, you intend to serve.

If you are planning on whole roasted chickens, plan on 100 pounds or more.

If you are serving wings, expect to serve a dozen or more per person.

If you are preparing legs and thighs, serve one each with about 25-30 extra.

If you are serving bone in or boneless breasts, again you will be serving one per person with 25-30 additional.

It depends too on how you are serving. If a buffet, purchase half again as much as you expect to serve, unless you will have servers to dish up portions. If you are planning a more formal dinner, one portion per guest is all you will need, but you might still want to purchase a few extra in the event of missteps (you don't want a guest to go without).