Check if you can get some help at the link below.
and from this link which will give you where there are located in your file system if you have it installed on one of your computers.
I have my original number that I put in to get the serial number, but forgot to write down the serial number. It's good to download on 2 computers, and I need the serial number to download on my new computer, but can't find it and don't know what to do help!
It is always helpful to include your Lightroom version and OS when asking for help...