If you purchased a physical copy from S&S Computing (pre-2015), the serial number was usually printed on a sticker located:
S&S Computing switched to digital downloads around version 4 and 5. Search your email archives (including spam folders) for keywords like: sewwhat-pro serial number
Disclaimer: This article is for educational purposes. We do not provide cracks, keygens, or illegal serial numbers. All trademarks belong to their respective owners. If you purchased a physical copy from S&S