The amount of water you drink per day varies by group. We find that we drink 1/3 gal or less. We don't cook rice or pasta. We don't drink coffee. We drink a lot of gatorades, diet cokes, and beer.

I agree with starting with 3 or 4 cases of 6 gallons and going from there. Before you buy again, you'll have a better idea of how much you need for the rest of the trip.