I have just quickly completed this mock-up, can you see the new first tab:

The tab has a small icon followed with how many files are within a the file list. This would be a useful feature for when I drag n drop multiple files to view in the viewer. Feature should be able to enable/disable if added to xnview.
If you disagree that it should be a tab then I should think that the developer could add a further option to choose how it should display being as a tab or as basic text elsewhere on the ui
Idea inspired by: Tab Counter :: Add-ons for Firefox