It depends on two things:
1. What shampoo are you using.
2. What pressure is your power washer
So. For normal shampoos like Meguiar's Gold class or similar, 2 to 3 onces per for 10 ounces of water seems to be the sweet spot. Try with your setup and see if the foam is to your satisfaction. For premium shampoos like Chemical Guys Honeydew, you can cut it half and put 1 to 1.5 ounces per 10 ounces of water for the same amount of foam.
The pressure washer influence your result. The more powerful the power washer is, better will the foam be. So if you have a 2500 PSI powewasher vs a 1500 PSI one, you will get more foam. This is why you need to experiment and adjust how much product to use.