Pumps for high pressure cleaning when used in the residential and vehicle cleaning industries typically range from 2 to 3 GPM for consumers and from 4 to 12 GPM for professional power wash contractors.
Water availability is always a key consideration when selecting a pump. If you are washing houses more than likely you will be able to run your equipment using a garden hose and their water supply so long as you do not have a pump rated higher than 6 GPM. Vehicle washing typically requires a truck/trailer mounted water tank capable of holding 300 gallons or more regardless of the GPM rating of the pump.
The only time you need higher gallons per minute will be when you are doing a lot of ‘flat-surface’ cleaning such as sidewalks & parking garages where ‘flush-time’ is critical or if you have your equipment set up for two-gun operation.