I've been thinking about this. I don't
think there is a (reasonably implemented) way to do what you want. You
can do a few things, but
sorting by the count of items in a category is tough.
If you want to have a [Director] category that displays the count of each item (in the Panes or category caption) then you can do this by using an Expression Category and then adding
GroupCount() in the Expression to Display setting. Like this, but with Directors:
Or, if you want a "Favorite Directors" Media View that only displays files by Directors which have more than X number of items, then you can do that instead. That's a little more complex, but with a Smartlist and the
~expand search modifier, it is possible. I have similar Views that show my "favorite artists" for music (those with at least 3 tracks rated >= 3 stars), and could explain how to do that.
But I don't think you can sort based on a GroupCount() because the grouping doesn't happen until after the sorting "pass" of the View filtering already occurred.