https://www.ctahr.hawaii.edu/hawaiirain/Library/Guides&Manuals/HI_Guidelines_2010.pdf
Near the beginning, it says that 30-60,000 people in Hawaii rely on rainwater so that might be where I got the idea that they all used it. It's been well over ten years since I collected my prepper files. While looking for the above guide, I also ran across a four page pdf on four rainwater harvesting systems run but the State. Maybe I read about that back in the day too. It's probably just not practical to get well drilling equipment to the smaller islands.https://health.hawaii.gov/sdwb/files/2019/07/GUIDELINES-FOR-REGULATED-RAIN-CATCHMENT-SYSTEMS-IN-HAWAII.pdf
Keep in mind that rainwater is much like distilled water in that it has no minerals so you'll need a supplemental source of minerals. Vitamins and tree nuts are the two I can think of off the top of my head.
One thing that hasn't been addresses is areas with little rain or very seasonal rain like the SW USA. I've got a folder called 'reuse' in my 'water' folder but there's only one 3 page pdf file, Laundry to Landscape and there's plenty of info to be found on the web for that; https://search.brave.com/search?q=Laundry+to+Landscape
All grey water can be reused for certain things but you don't want the chemicals in soaps building up in your veggie garden soils. Of course your first water need will be for drinking and if you have no water to drink, you won't be worrying about laundry or even bathing. As per AZ Granny, a gallon per day per person is needed just for survival which would be 16, 8oz servings. I'm sure a person could and many people have done with less but the gallon might also cover any food that needs water added.
If you wanted to store a year's supply, 6 - 55 gallon drums would be 330 gallons and IBC Totes come in sizes based on 55 gallons with 220, 275 and 330 gallon sizes being common.
Both plastic 55 gal drums and IBC totes can be purchased used most anywhere in the country but you'll need to make sure they contained food based products and not chemical. Some food based products are highly concentrated food grade chemicals and flavorings. I can get some IBC totes nearby right now that had Soy Lecithin in them. I once had some 55 gallon drums that had mocha flavoring and even after washing out with soap, they smelled like mocha/cocoa/coffee for a long time.
For free/cheap 1 gallon containers, these are the best.
I also drilled small holes in the Hawaiian Punch lids and used them for rinse bottles. It's amazing how many times those bottles can be squeezed without cracking. Unlike high density polyethylene that most 1 gal drinking water comes in, the PET holds up to UV rays and flexing very well and screw on lids are better than snap on. I've had them last for several years and always keep some on hand. When we lived off grid, this is what we used to tote water up from the creek and with the handles, four of them are easy to grip. I made it even easier by carving a slot at each end of some 1"x6" hunks of a branch so that I had a nice grip handle with a gallon hanging from each end. My wife and I would each carry four and the kids would each carry two which made it 10 gallons per trip. When laundry day was coming up, we'd make multiple trips but after doing laundry by hand a few times, the quaintness wore off so we started going to the laundry mat. I'd say laundry by hand was the most miserable job we ever did off grid and the clothes never really did get clean. I saw an article once about how the Seabees made wind powered clothes washers out of mostly wood. We've got a wringer washer now which I'm sure I could rig up to be wind powered. Not much wind in July/Aug when you could use it the most of course.
I've never lived or had a desire to live where there isn't water easily available. I grew up with a little spring nearby and then spent 25 years in Ctrl FL where you could drive a well by hand and put a hand pump on it because the water table is so high and then we moved to the rural Ozarks where we're surrounded by springs and also get 40 inches of rain a year.