MINIMUM NECESSARY - If you are getting a loan, the lending agency will have a lower limit. If you are writing a check for it, you get to decide.

If you were here in Idaho, wells are controlled by the state. When a well is finished a complete report is filed about depth, casing type and how many GPM the well could pump THAT DAY. The pump installer can tell you what pump he installed and its expected performance. Talk to the well driller and the pump installer about how the water level varies over the year and over dry years.

I have a 6" well 100' deep with lots of water at 50'. I water 2 acres 5 hours a day. I use 12 GPM 50% of the time thru a ground water source heat pump to heat and cool 9400 square feet. Normal household water is above and beyond that. We have no holding tank.

Check your water bill on what you use now and then double it.