id say 10g/day minimum and you might plan for 20g if anyone wants to wash hands. If you ever need it, you will be very glad for whatever you have. Even a couple one gallon jugs can make a power interruption or whatever much less stressful.

Rotate or otherwise pay attention to freshness if you want to drink it.

https://simplypreparing.com/weeks-worth-of-water-storage-for...

serious nuts do things like put 1,000gal underground cisterns in their water systems. and/or frequently drink from local water streams so as to maintain a tolerance for the local "wild" water.