My suggestion would be to carry at least 4 liters of water from Trail Camp to the summit. I think that having plenty of water is critical on this part of the trail; you don't want to be rationing water here because you've only brought 1 or 2 liters. There may or may not be a water source near one of the lower switchbacks, and there will certainly be no water source after that. On 4 past trips on the main trail I either drank all of the 4 liters I brought or came close to it by the time I descended to trail camp. Last year what worked for me for carrying water was a Hydrapak 2L seeker reservoir, a BeFree filter, and 2 Smart Water bottles; I filled up at Trailside Meadow just off the trail rather than at Trail Camp or the switchbacks. My plan is to do the same this year. YMMV!
Edited to add: here is a good list of water sources:
http://www.whitneyzone.com/docs/BobR/Whitney_water_trail.pdf
