240 million PCs were shipped last year, with about 10% being Apple. A negligible number run Linux. If we assume 5 years average life, that's still easily a billion active Windows devices.
That said, devices may not be the best metric. You mentioned users, which may use many devices. For instance, I use a Windows laptop at work, Windows desktop at home, Android on my phone.
I would use web server metrics, which are an approximate indicator of time spent on each OS.