Many "Old School" D&D type crowdfunded projects have around 200 backers, give or take.
If the Reaper Kickstarter is a reasonable function of the overall number of roleplayers, total, then "Old School" players might be 1/90th of the total of all roleplayers.
Quasi-recent estimates of the overall D&D player base provide numbers between 1.5 million and 6 million. The 6 million number shows up a few times, but one of those instances asserts that the number might be inflated. (See footnotes below.) The most recent estimate is 1.5 million current players, and it's also the most pessimistic example, which is good because this post need pessimism. It's not clear whether the 1.5 million number intends to include Pathfinder players, so I'm doubling it for these purposes, to 3 million role-players total.
So if 1 in every 90 players is an "Old School" player, then that's about 30,000 "Old School" D&D players total. VERY ROUGHLY.
Disclaimer: That's totally unscientific, and could be totally wrong in many ways simply by virtue of the assumptions made. For example, any of the source numbers could be wrong; the doubling of the 1.5 million to 3 million was a HUGE assumption on my part; not all buyers are going to be players, and the proportion of buyers-to-players could be different for "Old School" vs. other; a miniatures project may not be a reliable indicator for a non-miniatures project; that kickstarters are at all consistent in terms of the proportion of players to backers, for the applicable system; so on, and so forth. So the resulting number may be merely suggestive of a rough order of magnitude, and could potentially be too high by a significant amount (10x isn't out of the question).
Footnotes on overall D&D player base:
2000 Feb 7: "~ 2.25 million people" play tabletop rpgs monthly
2006 May 12: "Today, as many as four million people in the U.S. play each month, while millions more play worldwide."
2008 Jul 21: "“D&D” had about 6 million players worldwide last year, according to a survey by Wizards, though Rouse said the figure may be somewhat inflated."
2009 Apr: 4e court case documents: "Approximately 6 million people currently play D&D"
2010 Mar 23: "1.5 million players."