What is meant by the term "Shutter count"?
The number of exposures the shutter has fired is hidden in the EXIF information. Unlike the file numbering this is not resettable.
It can be read in photo mechanic directly, or in Photoshop by opening a raw file and look in "File info" under "file". Choose Advanced and expand "ns.adobe.com/exif/1.0/aux/." Read the number in "aux:ImageNumber: xxxx."
Source: Nikon Df initial shutter count