We had a non-backpacking adult with a 4×4 along, so she took a lot of extra water ahead to camp. Her husband and scout carried very little. Other scouts carried 5 quarts and drank as much as they could from a 6th quart before we left the car. Temps were not high (low 80s), but the sun was relentless.
Once we got to camp the cook water came from the car, so it’s hard to say how much the scouts would have needed to carry. They made pasta and did not conserve that water, which we wouldn’t have done if it was a water limited backpack.
Personally, I think I drank at least a 4-5 quarts in the 24 hours or so on the trail, and I probably should have drank more.
It was a fun trip. I heard a guy talk recently about hiking the JMT, and I asked him how much water he carried. He said typically one liter, but sometimes two if he wasn’t camping near a stream. That blows me away being San Diego based, where it’s almost always a dry camp.