The Google Stadia app has been the best way to guess at the current player base of the cloud streaming games service. Google wouldn't release any official figures yet, so that's pretty much the only way we have. That's because the Google Stadia app is still mandatory when using Stadia, even if you are planning to actually play on your TV or PC instead of your phone.
The Google Stadia app has finally crossed one million downloads globally. It had reached half a million only a bit over a month ago.