Datasource to poll for number of files in a UNC path?

You can do this fairly easily with a powershell:

$path = '\\##system.displayname##\path\to\count'
(get-childitem $path).count

If you need different paths / system, you'll have to provide a hash table of those relationships somewhere.  Either as a text file on a server accessible from the Collector, the collector itself, or hardcoded into the DataSource.  It will need to come in as a "Gauge" value rather than a counter.