How much oil goes in the pump?

@look slike it is not written down anywhere. In that case I suggest you change it as directed by the manual:

Pump Maintenance
Changing Pump Oil
Change oil every 50 hours or once yearly, whichever occurs first.
Change pump oil as follows:
1. Drain engine oil and fuel from pressure washer.
2. Use a 8mm allen wrench to remove black pump oil cap between high pressure outlet and garden hose inlet.
3. Tilt pressure washer to drain oil into an approved container until it drips slowly from pump.
4. Tilt pressure washer in opposite direction and empty "premeasured pump oil bottle" into same opening (a small funnel may be helpful).
5. Install black pump oil cap and tighten firmly.
Instead of using a "premeasured pump oil bottle looks like around~5oz" I get a small funnel and uses that to add oil until the level is just below the drain hole. It might also be useful, to measure the amount drained (pour it into an old measuring cup). That way you know how much you should add.