A set is 10 identical coupons, you put a (1) in the QTY box you will receive 10, you put a (2) in the QTY box you will receive 20, etc.
A set is 20 identical coupons, you put a (1) in the QTY box you will receive 20, you put a (2) in the QTY box you will receive 40, etc.