It is a difficult question to answer, since you do not need to register to use the script. Upon installation or first run, the script makes a request to my webserver for a file (either an image or a script). I keep track of those requests.
This is the information I have for 2014, for installations from Greasy Fork, OpenUserJS, and userscripts.org combined:
1/1/2014-3/31/2014: estimated 405 new and upgrade installations
4/1/2014-6/30/2014: estimated 456 new and upgrade installations