Hi health_scan_imaging,
If you are a Creative Cloud Enterprise customer then you can find the Serial no on Licensing.adobe.com and if you are a Creative Cloud for Team Customer then you don't need any serial no you can activate the software using your Adobe ID and password once you are a part of the Team users.
Moreover you can login to my.adobe.com and you can find your previous registered version serial no.
I hope this answers your query.
Regards,
Abhijit