Creating a new column to display number of songs

I recently found Mp3Tag and its amazing. But, I need help with creating a new column. I am trying to create a column that shows the number of songs that I have selected from a folder. Say, a folder has 50 songs and I select all of them, I want to have a separate column that shows which song I am currently editing. It looks something like this..

# Filename Title Album Album Artist Year Track
1 Somesong Somesong Queen Max 1989 04
2 Samesong Samesong Queen Max 1989 05

The 'Track' column is different than my '#' column here. The '#' column just shows how many songs I have selected to edit. I have tried adding "%_counter%" and "%_counter+%" to the Value and Field but, it doesn't work.

Can someone please help me with it? I don't know what to add in the fields of Value, Field & Sort.
Thanks a lot.

I do not think that it is possible to create a column.
But you see the number of selected files in the statusbar at the bottom right of the window.
You can also use the track-numbering assistant to renumber the currently selected tracks and add the total number to the track number.
You can then create a column that shows only that part of the track number.
Enter as value: $regexp(%track%,.*\/,)