I can only see how many "documents"(photo/video) with a folder after I enter it. I'd like to see the number of files in a folder without doing this. sort of like a a small bubble window at the corner of the folder thumbnail with indicating the number of files.

That would be too costly to generate, I am afraid. Getting those numbers is more work than just listing files: one has to check mimetypes to determine if they qualify as images/pictures.

I never considered distinguishing & counting by mimetype, it makes sense. I'm use to browsing folders with gwenview which solely have photos & video files.