When they're given out, they're sometimes given out to a whole 747 full of people. If you ask nicely, people will sometimes trade their $-off cert for your airport food voucher. Others will just give you the $-off coupon if they don't plan on flying NW in the near future. And some FTers have been known to scour the trashcans near the gates of delayed flights for discarded $-off coupons. So it's possible to get several at once.