Recent data suggests that the overall Windows user base may be bigger than I previously thought. To find out why, we'll need to do some math. And as with all math of this nature, it will require some educated guesswork. Microsoft will never give us all the data we need to arrive at a true figure.
So. In an announcement about some new Windows 10 store trends, Microsoft notes that "Windows 10 is now on more than 600 million monthly active devices, ranging from Xbox One consoles, tablets, laptops, Windows Mixed Reality headsets and more." We already knew about the 600 million figure; it dates back from late November and is probably still accurate given that most on-boarding Windows 10 users are upgrading and not new users.