You can buy them one at a time, and the coupon is applied one time at each check out. Buy the first one, check out, then go back in and buy another one and the coupon is applied again. Don't select more than 1 at a time, because you only get 1 coupon per checkout.
I just picked up three of them. It's nice to have extras around if someone needs one. The cheapest I've seen them on Amazon is $9.95 with free shipping, but I wasn't going to work about an extra buck or two.