Though we don't have sales numbers on Google Stadia's launch, an extremely rough approximation can be gleaned from the number of downloads of the mobile app, which stand at over 175,000 according to Sensor Tower.
The app appears to be effectively necessary to use Google Stadia, as it is the only way to purchase games and allows users to cast games to their browsers or TVs via Chromecast. While those who have not purchased Google Stadia can still download the free app from the App Store or Google Play, without an access code received from purchasing Stadia, the app is useless.
Sensor Tower has shared that 90.7% of all downloads of the Stadia app were on Android, while the rest were on iOS. The platform split may be at least in part due to the fact that Stadia can only be played on mobile via Pixel phones, making it a more appealing prospect to those already going through the Play Store in the first place.
Additionally, the US accounted for over 41% of all downloads of the Stadia app.