When I'm at a rest stop I typically get one carb (half a PB&J or a pack of crackers, etc.) and sometimes also whatever fruit is offered (half a banana, orange slice). I usually fill up one water bottle since the reststops are typically less than 15 miles apart, and I only go through one bottle between them. I would guess that the faster riders and bigger guys eat and drink more, but if you have an estimated number of riders perhaps you can go from there - a carb/protein option and fruit for each rider and enough water and sports drink to fill maybe 1.5 bottles per rider depending on how far apart the rest stops are and then some extra for the ones who eat/drink more. If you have same day registration, then you'll just have to give it your best guess and hopefully have some historical data to base it on. Don't open packages until you have to so you can return items that are unused. Better to have too much and return it than not enough. Nothing worse than a rest stop that has run out of food and drink!!