According to Ready.gov, a family sheltering after a disaster needs one gallon per person per day. Three quarters of a gallon is for drinking and a quarter of a gallon is for hygiene. The government suggests you store 1 gallon of water per person per day and keep 3 days of water on hand. They note that a person may need more in hot climate conditions or if involved in strenuous physical activity.
|Bottled Water photo by U.S. Air Force photo/Senior Airman Sonya Croston [Public domain], via Wikimedia Commons
The guide, How to Survive The Zombie Apocalypse, by Ben Jackson recommends two gallons per person for groups hunkering down. That seems reasonable. You'll also need to read up on water purification.
As I post this, the Atlantic hurricane season is starting. Hurricane season runs from June 1st through November 30th. Whether you are worried about zombies or hurricanes, a water stockpile is a good idea. At the very onset of a crisis, I recommend filling a bathtub or two in your home with water. This is a trick many Floridians do when a hurricane is coming their way. It will give you a reservoir of fairly clean water (depending on your tub!) to draw from for cooking or hygiene.
|Claw Foot Tub photo by Visitor7 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons