If your Beosound Stage is added to the Bang & Olufsen app, you can find the 8-digit serial number under "Product information -> Product settings -> About." The Bang & Olufsen app is available for download from the App Store and Google Play Store.
Alternatively, the serial number can be found on the packaging of the Beosound Stage. If these options are not available, the serial number can be found on a sticker behind the speaker cover. Instructions on how to remove the cover are available here.