Right. I should clarify that I meant for eating & drinking (survival), not necessarily showering or gardening
There's a lot of variation there. If you look at off-grid and deep-back-packing sort of sites, they'll give you a good idea.
For me, it came to 500 gallons per person, for Central Texas, to "stay alive through the worst historical drought I know of". We don't have any natural place to get rid of sewage (we're on a solid limestone hill), so that really complicates things for us.
Ready.gov suggests one gallon (4 litres) per person per day (https://www.ready.gov/water)
It's helpful to look at actual empirical usage. Van Lifers often give their own usage statistics. This article gives a 6.15 gal/day (23 litre/day) figure: https://musingsonentropy.com/2014/10/07/motorhome-living-one...
Household-level usage might be closer to 60 gal/day (225 litre/day) (1996 New Mexico water usage report, table of estimated usage).
Keep in mind that individual usage varies, emergencies (heat, illness, freezing) may change patterns, and that there is discretionary and nondiscretionary usage.
Don't undercount sanitation requirements -- regular bathing and flushing toilets will keep you healthy. It's possible to reduce those demands, but entirely eliminating them (unless methods are changed) is likely a bad idea.
There are these earthship houses in New Mexico that are amazing. They stay 62-72 degrees year round. They have orange and banana trees in the atrium. They reuse all water, the water that goes down the sink/bathtub is used in the greenhouse/entry/atrium, then it goes to the toilet, then it goes outside to a kinda marshland or wetland area where they have all kinds of greenery or maybe vine-grown vegetables and fruits (don't want black water on root veggies).
Its a bit sad too see how the mainstream system pushes people way from this way of living. "It can be difficult to obtain a mortgage for an earthship home. Many banks are unwilling to lend money for earthship homes, since there are few (if any) comparable sales in the area the lender can use to determine the home’s value."
I'd first recognised this by an aside in James Burkes's Connections, where adoption of the latten-rigged sail was delayed by over 1,000 years in the west by the fact that the financiers and insurers of sailing ship voyages would not permit ships with this "new" (it had been used throughout the Arabian region for centuries) and "unproven" technology.
(It's worth considering that much of modern banking, finance, business, and insurance practices came directly from the shipping world.)
More recently you can look at redlining in mortgage and small commercial insurance markets. Paul Baran, one of the co-inventors of packet-switched networking in the 1960s wrote of this in one of his RAND publications on the ethics of computer-science and information applications in the 1960s.
Want to renovate your house, but might need to tap the bank for a bit more mortgage ... think twice before you rip out the last bathroom and the kitchen - To mortgage a house without a kitchen or bathroom, many lenders will see it as uninhabitable and won't consider it suitable security. But I need the money to do put in the kitchen and bathroom ...