Foam cannon user here. As a user I have learned there are variables to consider. 1) If it's going to be a quick wash to knock off the "dust"...it works great. 2) Deeper/heavier soils or deposits will need more than the cannon to get a great cleaning done. 3) Using the foam cannon over a straight bucket style wash has worked great for me.
So I'll prep with an all around pressure wash to prevent rubbing in the big grit. Follow that with a good pre-soaking foam wash all over. Then using a clean bucket with various microfiber cloths, I'll foam cannon a second time to do hand wash. Finally a full rinse pressure wash to clear the foam, then a dry micro wipe down. I have tried using my shop air compressor to help with the wheels and other areas that "hold" water to prevent those runs that appear when you think you are done. I end up using/applying two bottles of the foam cannon while doing the above. So maybe 3-4 oz of wash maybe more. On a smaller vehicle I'd bet you could use one bottle of the foam cannon to do the same procedure.
Yes, it takes some time to do it all but it works great for me. Not to mention pressure washer PSI, your soap and water temp will determine how good the foaming action is applied from the foam cannon.