The serial number can be found on the product packaging.
If you no longer have the packaging, most products have a QR code that can be scanned to get the serial number.
Bluetooth Dongle serial numbers can be found using the Sennheiser Dongle Control Software.
Product Packaging
Wired and Bluetooth Headband Headphones
Wired and Bluetooth In-Ear Headphones
TV Listeners
Bluetooth Dongles