Last night, we found that we had no water in the house for several hours. Thanks to Twitter (and not the Cleveland Water District website), we found out that it was because of a water main break that has affected thousands of residents - and sending many of them to the 24-hour grocery store to stock up on drinking water.
The water came back on this morning, but residents are advised to boil their water before drinking it or cooking with it until 10 pm (and again, the only reason I know this is by Twitter).
Fortunately, we did have some emergency drinking water in the house, next to the box of MREs. We had two and a half gallons.
The general rule is at least one gallon of water per person, per day of expected need (with a three day minimum but two weeks is often recommended) - 2 quarts for drinking and 2 quarts for food preparation and sanitation. If you have pets, allow 1 quart per day for each dog or cat. Stored tap water should be rotated every 6 months. Prepackaged bottled water should be rotated once a year.
So, really, we should have at least 9 gallons, but probably even 42 gallons. That is a lot of water to have on hand.
In a real emergency, one could also get water from the water heater, which typically holds 30 and 60 gallons of water.