I have three batteries, but only really tend to use two for a proper clean which requires a lot of rinsing. Two are the 4ah official batteries, another a third party 5ah battery off eBay that definitely doesn't last as long as the official battery despite claiming a higher capacity.
The proper clean has a lot of use of the PW because there's rinsing the wheels after pre-wash, rinsing them and arches after the contact wash, applying snow foam via lance, rinsing, contact wash on bodywork, rinse. Might the occasional additional rinse if using chemical decontamination products.
I don't think I've ever used more than the two batteries, I have three because the 3rd party one was initially temperamental and Greenworks had a black Friday sale on their direct to consumer website.