There's quite alot of us, and mostly the veteran Yoshi mains know how to, but hardly implement it into their game.
In 2008-2010 I hardly used DR, I was a wifi warrior and, cause of lag, the best thing I tried to practice everyday was B-reverse and Wavebounced Eggthrows since DR isn't needed all that much in incredible amounts of lag.
I didn't actually try DR unless it was offline friendlies with people around my old neighborhood or just to screw around and try it when the opponent is coming back after losing a stock.
When I joined the competitive scene in 2010 around May, I started learning more and more about Yoshi, including DR. I don't use it alot like I should be, but at most... I'll actually pull it off when I want to about 3-7 times and get rewarded for doing it instead of punished.
Anyways, sorry I ranted a bit. To answer your question, at most 80% of us know how to DR, but around 50 or less % actually use it all the time in their tournament matches.