In ndMirror 1.1 on the cabinet page you can tell from the cabinet page if a sync has completed. If it has, then the number of documents shown there is the total number as of that sync in the cabinet. This is constantly changing, so the next sync ndMirror will start another series of syncs (it could take more than one day if you added several tens of thousands of documents) until they complete. Once they do, that number will be updated with the new date.
In the web, while there isn't an easy way to do this, an advanced search that includes all the document types gets a pretty accurate number of the total documents on the system like this one here: