This article is based on work of Dr. Lauer and his group at the Cleveland Clinic. I believe the original work was done for older patients with preexisting heart conditions. His work was updated on a much bigger group in a study funded by the American Heart Association. In that study, a summary of which can be found here http://circ.ahajournals.org/content/104/16/1911.long which covered a much bigger group, more representative of the general population, the "magic number" was 18.

PLEASE don't let this thread digress into an internet "mine is bigger" fest. No one cares if you have a big number, and if everyone who has really good HRR starts posting how great they are, other people with smaller HRR - the ones who would probably benefit most from the discussion - will shy away.