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This file demonstrates that the Count() function does not seem to work in list box Expression searches.
In fact, it should not work. Where did you get this information?
Search box is not intented for aggregate functions like count or sum.
See Henric's blog post for using sum:
The Expression Search
Very interesting... and, it works!
I never have seen this feature before... You must make the search on a dimension field (for your example, I used the "Name" field). Take a look: