The how to start a team flyer (
http://usfirst.org/uploadedFiles/Rob...HowToStart.pdf) recommends 2-3 engineers, but only 2-3 other adults. The same recommendations are in the How to Start a FIRST Robotics Competition (FRC) Team document (
http://usfirst.org/sites/default/fil...team%20v2.pdf). The FRC Handbook (
http://usfirst.org/uploadedFiles/Com...20Handbook.pdf) mentions the need for both engineering mentors and parents, but does not give specific recommendations on numbers.
All of these are from the "How to Start a Team" page on the FIRST website (
http://usfirst.org/roboticsprograms/...rt-an-frc-team).