A lot of water, or chlorine bleach.
Something I thought of earlier, buy plenty of dried beans, rice and macaroni, but also, be sure to buy cans or can plenty of vegetables; you can use the liquid from the cans to cook the dried foods, saving on bottled water.
Bleach degrades to salt water over time.
I have bags of pool shock - can be diluted to make bleach and takes up less room in the storeroom.
Chlorine bleach has a effective life even in sealed gallon jugs of just over a year. The chemical itself is unstable it disintegrates to salt and water over time. For long term chlorine you need powdered chlorine they same type used for clorine tablets in pools.
Chlorine bleach will lose most of its sanitizing potency within 6 months. And rapidly degrade in the ensuing months. So after a year your bleach is down to 20% potency.Store the chemicals (Calcium hypochlorite) to make your concentrated chlorine solution. A small amount of solution is mixed with an amount of water to then mix with a larger amount of water which equates to household bleach. But now this homemade bleach has the same shelf life as store bought bleach. The efficiency of making the chlorine solution is that you can just make small amounts for a few weeks/months usage.
1 pound bag of good product (then made into the aforementioned chlorine solution--then mixed with water=bleach) is enough to treat over 10,000 gallons of water.
Do a search...numerous articles on the mixing amounts, ratios, safeguards etc.