The lookup is the best answer but howsomever.
Most smaller BS and some larger use 20 oz.. Lawnmower oil is readily available in this size container. Where they came up with that number I'll never know. Or you can buy the usual quart with the measuring stick conveniently provided. Gone are the days of the metal quart oil can with no markings.
For years I have filled engines to the top of the fill plug, which is more than 20 oz, but that 's what I did. Never regretted it.