I know how to check how many wishlisters want a certain book.  How do you find out the number of books that are posted to pbs? I know it can be done but don't know how.

Add the book to your Reminder List.  I think that's the only way...

Just add the book to your reminder list.  It will tell you how many are on here.

The number of people on WL for a book is right on the book data page.  Just pull up the record for any book and you'll see if there is a WL for it or not.  If you see the "Order This Book" button, then it's readily available in the PBS system.

To see how many of a specific ISBN are posted in the system, add the book to your Reminder List.  When you view that list, you can see how many books are posted currently.