You get X number of snacks/person/night of stay. ON the DxDP, you get 3, on the regular I think you get 1. So, you get a total, say you're on DXDP staying 7 nights you get 21 snacks. You hand over your card and the CM swipes it, removing 1 or more snacks from your total depending on how many you buy. If you're buying snacks for yourself and all members of your party, you only need to use one card. Each card for the ressie is encoded with the total number of snacks for all guests on the ressie.
Anyway, after two times using the DxDP, I learned there are more bang for your buck snacks than using a credit for water. We've gotten ice cream, ice cream floats, cup cakes, fudge, giant peanut butter cups, etc with our snack credits. The Main St. Confectionary has all sorts of selections. Each country in Epcot also has some snack offering. Have fun!