Very useful and a welcome addition, but looks messy, and makes it hard to read at a glance.
I agree with this bit, and this has been on my wishlist for a while too.
Since you're not actually looking for the genre/artist/etc with the most/least number of tracks, or want to sum the numbers together, then the numbers doesn't need to stand out.
I disagree with this bit. Sometimes it is useful to see which artists, genres etc are most common.
Also, right-aligning the number could make people believe they can sort the the track browser columns by number of tracks.
And lastly, I think it can be difficult to achieve programming-wise.
These are both true, which is why it shouldn't be fixed by just right-aligning the numbers - the number should be shown in their own column, which would allow them to be sorted (not sure how that would work though, because you don't want to split the header row - right-click menu option perhaps?).
Here's what it would actually look like. I think it's much nicer:
Or even this:
Another possibility would be displaying track duration totals or file sizes.
Bee excellent to each other...