Microsoft launched the Windows Insider program back in September of 2014 and as of today they've logged their one millionth piece of feedback from testers.
The news comes from Gabriel Aul, an engineering manager at Microsoft responsible for the Windows Insider program:
Wow! We just received our one millionth piece of feedback from the Feedback app! Woohoo! #WindowsInsiders
Windows Insider first launched for testing of Windows 10, followed by a release in January 2015 of the Windows Insider app for Windows Phone. While Microsoft hasn't yet released a full Windows Insider preview of Windows 10 for all Windows phones, the technical preview has been released for several devices so far, and offers a "slow" or "fast" track that allows users to manage how quickly they get releases (and how many bugs they're likely to encounter in the process).
Source: Gabriel Aul