Lots of good info already from people. Here is my situation.....
We use about 1200gl per month in the non-growing season (Oct-Mar) for two adults, 2 dogs, and 2 shop cats.
Growing season (Apr-Sept)we will average about 1000gl+ per week for personal and 490sqft of garden beds.
I have a 1200sqft metal building/shop that I catch rainwater/snow melt from. 1" precipitation per 1000sqft of roof gets you 600gl of water (approx).
I have 3x2500gl, 2x1500, 1x500 for 11000gl of storage plus another 1400gl in the tanks I haul with. I Keep minimum of 7500gl on hand in the summer and let it go down to around 4500gl as we head into winter. I adjust where water is at to leave storage capacity open for the snow melt in winter or the rains during monsoon season. In two winters I've been here, all of my tanks were full or snow melt when growing season started. No matter how much it rains or snows, if you've got nowhere to put it, you're SOL. By the end of this summer or early in spring I will be helping my Father-In-Law built a 20k gallon cistern to share on their property. They are 1.5 miles the other side of the valley and have 6 horses. We've been here two years and have already built up a solid infrastructure. But more water storage will never hurt.
In winter I haul 1100gl at a time on my trailer and in summer I add an extra tote to the bed of the truck to keep it down to around 3-4 trips to town a month to get water 1400gl at a time.
I researched a well on my land. 300'-500' @ $30k-50k with no guarantee I won't have to go deeper in the future or even if I do whether there will be usable water. For that kind of money I can haul an awful lot of water and catch a lot when it rains.
I am also good friends with the rancher in this area and in an emergency, he has two different wells within a mile I can use.
All of the water going into our home is 10 micron GAC, 5 micron paper, then UV filtered. Garden and cloths washing is simple paper filter for solids.
This time of year there is another 275gl tote in the bed of the truck. It costs me 1 cent per gallon to get water in town here.