How to find old devices I’ve owned

You can try by logging into...

https://support.apple.com/my-support

Login with your Apple ID and Password. If the device you are looking for was registered in your Apple ID then it might appear on that site. Depending on how old the device is, though, it may dropped off.

Why do you think you need the serial number if we might ask?

hello,

if you had the app Mactracker and saved your devices with the serial number, you'll find it. you can download this app right from the App Store.