Thanks DTC and WER,
... "Classical Music # 30271" telling me that there are 30271 records who contain, as one of the values in the genre field, the tag classical music...
Meaning you just want the count of the files that have that tag, which is exactly what I said before.
So I would say Zybex's first answer was correct for what you actually first asked, because you said you wanted a count of albums. His latest answer is correct if you want files.
Moreover, as I said before, this is even more easily done in a panes view, if you want to do no work at all: click on the category you want, and at the bottom of the window in the status bar, MC shows you the count of files that have that tag. No expression work required. It's easy to overlook, but MC counts and summarizes everything you select that way.
Finally, if you want a simple list, instead of doing a Category view as Zybex described, do a panes view. And add a category as described before:
Name: Genres
Expression to group by: [Genre]
Expression to Display: [Genre] (GroupCount())
That will give you a nice list in one place without using the tree.