The serial number for the UMIK-1 can usually be found on a sticker stuck on the microphone body. In case it is missing, you can always go back to the packaging box and the serial number should be available on the side of the box.
If you are still not able to find the serial number in both places and you purchased from miniDSP, we "might" be able to recover the SN for you based on our database. Please contact our team for further support.