That 350 million figure is still being bandied about. I seem to remember the lead up to a recent event where that figure was somewhat discredited...
...and it might just be that MS's figure isn't entirely above board either. AIUI, 350 million is for downloads/installs to all device types. So it's not just for PCs (although the vast majority of that figure will be, as the Surfaces and Phones probably don't amount to much).
Also, we don't know if that 350 million includes downloads/installs that have been rolled back, or for which PCs people who upgraded decided to wipe it out and start again with something else.
So basically, we don't know what the true active user base of Windows 10 is. I don't think MS do either. But I'm pretty sure it's a fair bit less than 350 million.