You can use the map D/T-tool and a custom filter for this, the easiest way is to use a name of somebody that haven't logged any caches in the area (for example a zero finds account or somebody that hasn't been to the area), the issue however is that there are over 1000 challenge caches in the area so you would have to make multiple searches and use a Vgps and some manual work to figure out the actual numbers.