You need about 15 pounds of cut lettuce to feed 75 people, resulting in a portion size a little above 3 ounces. Taking into account the loss of about a quarter of each head of lettuce, you'll need to buy 20 pounds of heads to yield 15 pounds of edible greens. If you use leaf lettuce or romaine, where you lose about a third of each in preparation, you'll need about 23 pounds to yield 15 finished pounds.