Burn To The Brim Help
  Percentage filled will show you how much of the medium will be filled with the currently best selection.

During the scanning of folders, # items shows you how many files and folder it has scanned, in batches of 1024 (so for normal use you won't see it at all). During calculation, it will show the number of files/folders it has used (you may remember that this number will be minimized).