From the help center:
How can I tell how many copies of a book are in the system?
If you want to know how many copies of a book are currently available in the system you can do a search for that book and put it onto your Reminder List. On your Reminder List, beneath the title, you can see the number of copies currently available in the system.
If you plan to use this information to guide you in posting books, you should know:
If you Post the book quickly after checking the Reminder List, yours will most likely be the next one in line.
However, knowing what 'FIFO number' your book is in the system would not be very helpful in predicting how quickly it will be requested.
You would need other information about the dynamics of that particular book on the site.
For example, if you have an obscure book that is the only copy in the system, it could take months to be requested. If another book you have is #33 in the system, but the book is requested very often, it could be requested in just a few days.
If you knew only these FIFO numbers, you might misinterpret them and take #33 off your shelf and donate it, while keeping #1 on your shelf, when you really should do the opposite. this information would not be terribly helpful, and could easily be misinterpreted to mean that a low number would 'move quickly' and a higher number would linger on your shelf, which is often not at all the case.
We are working on a 'popularity index' which will better indicate how likely a book is to be ordered within a certain timeframe. It is on our long list of ToDos. We hope to have it available to the membership soon!