Fiveo
If you are doing just washes you can get 10 to 15 washes maby more. The more dirt on the car the more water you will use. You dont want to have your pressure to high if you can control this. I keep mine low and I don't run the motor at a high speed. When I used to do alot of maintenance customers (cars I would wash every week and wax every 4 weeks) I did not use alot of water. Some times all that was needed was a rinse, light cleaning of the rims and that was it. I could probably get 20 cars at this rate.
Do you use DI or RO water?