The guideline is one gallon per person, per day, for drinking. That’s the minimum.
During an emergency you will also need water for washing your hands, brushing your teeth, washing dishes if you don’t have disposable plates, washing fruit and vegetables, and other sanitation needs. For example, a 72-hour emergency pack for a family of four would need 24 gallons of water for drinking only, but more would certainly be welcome. The basic 24 gallons of water will weigh about 200 pounds and will require 3.2 cubic feet or more of storage space.