Water is the supply that matters most and the one people store least, because it is heavy and dull.
How much. National guidance converges on roughly three litres per person per day for drinking and basic use, and more in hot weather or for anyone unwell. Your country's authority states its own figure, and that is the one to follow — the period differs too, from 72 hours in Finland and Lithuania to a week in Norway.
Do not forget the rest of it. Washing, cooking and sanitation take several times what drinking does. A stored bucket for flushing is unglamorous and transforms a week without supply.
Storage. Shop-bought bottled water needs no preparation and keeps for the period on the label. Tap water in clean, sealed food-grade containers should be rotated every six months — write the date on the container, because you will not remember.
The bath. The moment you have reason to think supply may be interrupted, fill it, along with every pot and bottle. This is the single highest-value thing you can do in the first two minutes of a warning, and it only works before the pressure goes.