Thank you for your reply. I immediately added it this way.

Hi,
You can Right click on Column header -> Other -> # to enable it.

But you are right it is missing in Choose columns added as https://www.ventismedia.com/mantis/view.php?id=16590

After customizing columns of playlists, the sequential order of tracks (#) isn't available anymore. Thus, after (accidently) sorting tracks in a playlist, the original order of tracks cannot be reached anymore.