Since OpenEMR is open source and supported by numerous vendors, companies, and volunteers, it's very difficult to accurately collect this information. It's downloaded more than 4000 times per month, and the project is confident there are thousands of installations in the US and across the globe. There was an attempt in 2012 in this article to quantify the number of installations:
http://www.openhealthnews.com/hotnews/openemr-continues-grow...
I'll note that there are (possibly out of date) estimates at the bottom of the Stage III page of ~31 developer months of work to be fully ready for Stage III and an estimate of $22k to cover the cost of certification testing. I suspect the testing hasn't gotten cheaper in the past 2 years, and I don't know how much developer effort has gone into MUIII.
On a quick scan (the spreadsheet file was 300MB and it looks like I wasn't able to load everything on my computer at work), there are at least 300+ OEMR attestations (this is the non-profit organization that represents OpenEMR for MU certification).
Will plan to look into this further and thanks again for pointing this out.
