You'll figure it out as you go. I use a 3-liter Platypus bag in my pack, and a couple of soda bottles as backup. Learning how water much to carry at any given moment is probably one of the major mental challenges you'll face on the trail. As a general rule, I tank up when I'm about to climb up to a long ridgeline -- water is usually found "down low" and not on the ridgeline. The maps and guides will tell you where water might be found. Sometimes the shelter registers and other hikers will have further advice. Another tip -- "camel up" (fill your belly) where you do find good water.