Just a guess on my part - browse to c:/DMS or whatever file you use. There is a folder DETAIL, right click and select properties. That should show Contains XXXXX Files,  zz Folders.. Best guess XXXXX is the number of documents. 

You can highlight all clients > File > Export > A folder on my computer      That will create windows folders with all the client files in regular plain old windows layout. You can then count them. This is also useful for one client to give them a copy of the files you have in DMS