If you purchased a physical copy, the serial number is typically found on the DVD sleeve or a sticker inside the box.
: If the program is currently installed and working, you can find the serial number by going to Help -> About (though this menu may be inaccessible if the software is asking for activation). Recovery Options for Lost Keys
A Pinnacle Studio 16 serial number is a unique code that is required to activate the software and access all its features. It's a 25-character code that is used to verify the authenticity of the software and prevent piracy. Without a valid serial number, users are limited to the trial version of the software, which has limited features and functionality.
If you are looking for a budget-friendly video editor with a similar feel but modern capabilities, you should look at (or the current version) or DaVinci Resolve (which has a powerful free version). These modern editors support 4K, modern graphics cards, and current video codecs.