I think the 15 million figure is too high. According to Wikipedia 20 million have played D&D at some point, and 5.5 million (as the last poster said) currently play some edition of D&D. Now to what degree, who knows. And you have to include a sizable number of folks that rarely play, or are on hiatus from playing, but buy books and keep up with the industry.
So if the 5.5 million figure is close to being right, and if you add in another 10% who don't play but buy books, and then add in another 30ish% of that figure for other games, you're looking at about 8 million, give or take a couple million. So my guess is that we're looking at a range from 6-10 million.