The only serial number requirement for Numbers is to use one to convert the iWork '09 thirty day trial to the 'full' version.
If you are getting a message requiring you to enter a serial number, it's because you earlier downloaded the trial edition, and the thirty day period has expired.
The cure is to delete iWork AND the support files that the iWork applications created, then reinstall iWork (or the individual applications, if you purchased them separately).
The easiest and safest way to uninstall the iWork trial is to use the uninstall iWork '09.zip file found in Yvan Koenig's Box account. Click the link, then go to for_iWork'09 > other_iWork'09 items > uninstall iWork '09.zip.
Unzip the file, open it with the Script Editor and run it.
Note that if you received a disk or disk image to do the installation from, you will require that dmg or physical disk to reinstall. If your purchase was through the Mac app store, you will be able to re-download from your 'purchases' list there.