As of December 17, more than 1.5 million people have registered as "Windows Insiders" who can test Windows 10, according to Microsoft officials.

Microsoft

Of those 1.5 million, 450,000, about 30 percent, are "highly active" testers putting the preview build of Windows 10 client through its paces daily, officials said in a new blog post.

Officials said Windows 10 testers are using Windows 10 test builds more actively than participants in preview and beta programs for Windows 7, 8 or 8.1. Microsoft has fixed almost 1,300 Windows 10 preview bugs reported and/or up-voted by testers, officials added.

Back in mid-October, shortly after Microsoft released the first public Windows 10 Technical Preview build, Microsoft said 1 million people had signed up as Insiders to test Windows 10.

When they released its second preview update to the Windows 10 Technical Preview in November, Microsoft execs said the next officially sanctioned Windows 10 preview update would be in early 2015. There have been a couple of unsanctioned leaked builds since then, the latest of which (Build 9901) shows off some of the UI changes Microsoft is expected to make part of its upcoming consumer-focused preview in January.

The reason for the lapse in what was shaping up to be monthly officially-sanctioned test builds, was explained by Gabe Aul, head of the data & Fundamentals Team in Microsoft's Operating Systems Group (OSG):

"We've been very hard at work putting together a great build for you that includes a bunch of new features and improvements. As all of those payloads came in we needed to stabilize code, fix any integration issues, and ensure all of the new UX is polished. We're really focused on making the next build something that we hope you'll think is awesome. In fact, just so that we have a *daily* reminder to ourselves that we want this build to be great, we even named our build branch FBL_AWESOME. Yeah, it's a bit corny, but trust me that every Dev that checks in their code and sees that branch name gets an immediate reminder of our goal."

(FBL stands for Feature Build Lab. Or actually, maybe it's Feature Branch Level. )

Microsoft is expected to release its next Windows 10 Technical Preview -- with more consumer-focused features enabled -- in late January, possibly on January 21 during its Windows 10 "Next Chapter" reveal event in Redmond.

Microsoft also is expected to show off at that event its Windows 10 mobile preview, which is expected to run on Intel- and ARM-based tablets, as well as Windows Phones. Microsoft's OSG is hustling to make the mobile preview available to testers by January 21 or shortly thereafter, sources say.