Since I was researching how to treat heart disease, Google quickly directed me to Dr Esselstyn and Dr Dean Ornish. I started following Dr Esselstyn's advice quickly. Within a few days, I came across this site and got to hear Dr McDougall's message. I eventually bought many of Dr McDougall's books.
It would be great if everybody who is considering a plants only diet got to know Dr McDougall. His books are written very well and his message is simple and common sense. And more importantly, his advice is much easier to follow long term. And it is much cheaper.
Instead of estimating how many McDougallers are there, a better count would be of people following a whole plants only diet with little added oil. I think based on how many copies Dr Greger has sold and how many Dr Esselstyn/Dr McDougall/Dr Neal Barnard/Dr Campbell etc have, the numbers will be in millions worldwide. More importantly, this is a growing population.
In India alone, there is this doctor: Dr. Bimal Chhajer. He has scores of centers all over India treating thousands of patients using the whole plants only diet.
http://www.saaolheartcenter.com/about-dr-chhajer.php. I heard people in China are also hearing this life saving message.
Since many ancient and large civilizations (China/India/Africa for example) used to follow many tenets of this diet, they may start comprising larger and larger share of adherents of this diet as time goes on.