[GeoffR]

If you want to find the number of books currently on the device that are marked as finished (usually different to the number that the stats show as finished, because the stats include books that have been read and deleted, and don't include ones that were just marked finished without actually being read to the end) one way is to set the library filter to Show: Finished and work it out from the library page count.

e.g. on my Glo the library shows 5 books per page in list mode, so the number of finished books is 5 times the page count less the number of missing books on the last page.