Required minimum of 20? Nah . . . you can do it with less CFM than that, it just takes longer and sometimes multiple passes at each zone (if you are paranoid like me). I have a 33 Gal Craftsman that I got on CL years ago and it's rated at 8.6 cfm @ 40 psi. Takes a little time but it works fine.
I've heard people say they use even smaller compressors than that. But I tried it with a 15 gal I have and it just didn't have the oomph for my setup. That's why I had my eye out for something like the above.