Yep, and every time we flush it is a couple of gallons. My scenario of 3000/1440 gallons per year would require you to reduce sanitation (that 1000 per month that the washing machine uses plus flushing (at 1 flush per person per day would be about 400) and 6 gallons per load dish washing ~100) down to 250 gallons per month...
But that is still a lot of water to try to store securely.
I like
@LadyLocust point about using your canning equipment, stock pots, and empty canning jars to store water, I know that mine could hold close to 50 gallons.
Not everyone can be as fortunate as
@Sourdough and have running water readily available; but those of us who don't have a clean running water resource will need to think about gathering, filtering, treating, and storing a lot of water.....