Go ahead and get the book, MOUNT WHITNEY: Mountain Lore from the Mountain Store by Doug Thompson. It answers every question you can think of. There is also a list of places to find water on the trail on this site. Here it is:
http://www.ridgenet.net/~rockwell/Whitney_water_trail.pdf
As far as how much, everybody is different, but most say 3 liters after Trail Camp. Just check out this site and you will find lots of info.