While it's possible to show a count of features in the legend, and it is possible to only show features visible in the map extent, it is not possible to only count features in the map extent. the 'Only show' checkbox has no bearing on the total counts represented -- that is, if you use this, it's still showing you the total count of features, including those not visible in your map, which could be very misleading. This would be a welcome option so that I don't have to manually count, convert to graphics, and correct the numbers ArcGIS provides.