You will find my comments about Moodle 2 usage at the top of this reply, and my best guess as to how to determine usage in 1.9 at the very bottom.
If you go to http://moodle.org/stats/ you will find the "stats" - You can easily state that all Moodle 2 installs have access to DragMath as it is installed as part of Moodle core. If there are 70,000 sites (dubious) with 10% running Moodle 2 (dubious) that means 7000 sites have access to DragMath. My guess is that you may have many more sites running Moodle 1.9 with DragMath, but there is no telling how many. Perhaps more importantly, though, is the number of large Moodles in which DragMath is being actively employed by students and teachers. Mauno may be able to offer some insight with respect to downloads of SEE for 1.9, as that would reflect aggressive use of such tools, which have always encompassed DragMath.
Discussion of DragMath initially consumed a bit of conversation on the Math Tools forum as you will note. As we were able to make installation easier, the number of requests for assistance dropped precipitously (most actually resulting from people not reading the instructions, lol.) I suppose you could make an argument about the number of users based on the discussion in 2007-2008, but ti would not be very accurate.
I tried to retain the full history of DragMath and Moodle in the Documentation here, http://docs.moodle.org/19/en/DragMath_equation_editor and here http://docs.moodle.org/19/en/Talk:DragMath_equation_editor , the later reflecting some of my thoughts on transitions etc. You will note there is little participation in the docs - that may be because I'm an ass or because I tried to incorporate any issues appearing in the forum in the docs as quickly as possible (or both, lol) but they won't provide much in the way of insight as to usership.
John Isner assembled the first integraton package (I believe in Fall 2007) and he sucked me in when he decided he had had enough Moodle some months later. There were efforts afoot at that time to pull the DragMath integration into the Moodle CVS system, and though I didn't think it would work because of the way Moodle insisted serving things up, I was a good sport and agreed. It quickly turned into a debacle and I repackaged the code with new instructions and directed users to use a new download link. made available by download. There is no way to track downloads from the old link.
Subsequently, working with you and Alex, we did some tweaking to cobble up a Moodle-centric package, that would remain current as well as address some bugs and feature requests (one of which is still outstanding, lol....) As a result, all download of dragmath for Moodle 1.9 would involve a download via http://www.dragmath.bham.ac.uk/moodle.html which then redirected for downloads back to sourceforge! So if you have historic data on hits there, this might actually give you quite a good estimate of the number of installations of 1.9 Moodle where DragMath is installed (and will offer some kind of differential between total downloads and downloads for Moodle, though the forum posts suggest that not all users followed the insructions, so some may have downloaded without going through the moodle.html page.) The question is simply whether you have dumped the data or not (it is a bit old at this point) but if you do have it simply parse the hits for 2008-2012 by unique ip address and count them up and Bob's your Uncle.