In the new dutch compain to educate people about prepardness, they recommend 4 liters. Which interestingly generated complains from a watercompany, because apparently there is a law that says you should have 3 liters...

But in reality, the amount per day depends on your needs:
2-3 liters (more in hot conditions) for comsuption.
0.5 liter to brush your teeth.
2-3 liters for cooking and washing food.

This means no shower, no washing hands, no water to clean anything, nothing to flush your toilet, etc, etc. A shower takes about 43.7 liters and flushing toilets for one person a day 35.8 liters (dutch average). This does not include washing hands, washing clothing, etc. So how much luxeury do you want when something is going badly? Answer is up to you.