Safari subscriptions are based on a total number of slots used, not the total number of books on My Bookshelf.  The majority of books in Safari take up 1 slot, however, some take up 1/2 slot, 2 slots or 3 slots.  Slot values are a factor of the retail price of the print book.  Click on My Bookshelf to view the slot values of the books on My Bookshelf.  Or, to determine the slot value of a book that is not on My Bookshelf, simply click on the book cover. You will always be told the slot value of a book before you confirm the addition of a book to My Bookshelf.  If you need assistance please contact Customer Service.