If you received Virtual DJ LE (Limited Edition) with a controller (like Numark or Hercules), the serial number is often on the CD sleeve or a sticker on the bottom of the controller.
Serial numbers for Virtual DJ 7 are strictly personal and linked to the account used at the time of purchase. You can typically locate your key in these places: Serial Number Of Virtual Dj Pro 7.0.5
Check the email account you used when you originally bought the software. If you received Virtual DJ LE (Limited Edition)