For portable emergency kits , the optimum amount would be one gallon per day, per person. One way to store water is to use beverage containers that would normally be thrown away. Plastic soda pop bottles and glass juice bottles have all been used in the past, some more successfully than others. Some times cleaning used plastic containers is difficult because plastics tend to absorb whatever is stored in them. In the case of milk bottles, the absorbed milk may leak back into the water from the plastic and sour the water.
Harmful bacteria can grow in such an environment.
Therefore, water should not be stored in milk containers. Plastic containers that have not been used before are usually better than used containers. These should be food grade plastics that are made for water storage. Good containers are readily available in a variety of different shapes and sizes. One thing to remember is that water is heavy and anything with more than twenty gallons in it would be very difficult to move.