How much water you need depends on a variety of things, including weather, exertion, how much you weigh, etc....
I've gone up with several women, all of us 120-140 lbs, in both July and September. Three liters was sufficient for each of us. One woman who only carried 2 liters ran out, and had to be bailed out by me and a friend. Being able to purify water on the switchback spring does add a comfort margin -- it's been there when I've hiked, though folks say you may not be able to count on it.
I carry a 2L camelback, and a 1 quart Nalgene. I use the Nalgene to purify water with my Steripen, and to carry 1 quart of electroytes (Citomax) between Trail Camp and the summit.
