A Lot! For a lactating cow, figure 50 gallons/head/day. A dry heiffer might be only 10 or 15.
You really dont want to haul water to your cows. Is there any way to design you pasture layout so that the cows can always walk to a water source? Sometimes my cows have to walk 1000 feet to their water, this is good excercise, and a healthy thing. I would never want to have to haul water to lactating cows. I do bring water in buckets to a heiffer that is teather grazing sometimes, but even at 10+ gallons/day, it gets old real fast.
Having flexible paddocks is really advantageous for many reasons. I have very few permanent
fence divisions within my property, so I can change the location of grazing paddocks and paths to better manage my pasture. In a more permanent setup, the paths end up so trampled that they become sacrifice zones. Also, your pastures will grow differently during different seasons, so varying the paddock size and location allows you to target your grazing in a way that is best for the animals and the pasture.
good luck!