This article explains in detail how to identify counterfeit Lightning cables:
https://support.apple.com/en-us/HT204566
Provided it‘s still up to date, the serial number should indeed have 12 digits, not 13. i recommend you take a closer look at the lightning connector itself - it seems to be relatively hard to build a cheap knockoff of this connector that looks and works just like the original... So do compare its details with the pics in the above article.