As a person in the catering business, my rule of thumb is to allow about 6 oz of meat per person if only serving one meat. 6 oz Chicken Breasts are readily available at super markets, they are the most common size and usually can be bought in 5 lb bags. At 6 oz per person you would need about 9.3 lbs, and to account for a person eating more than one which will happen very rarely, or maybe overcooking a few- two of the 5 lb bags should be plenty.
