I don't want to waste my time and my space at home posting and holding books that hundreds of people have already posted.

Is there a way to see how many copies of a book have already been posted before I list a book I have?

IIRC, add the book to your reminder list it will tell you how many copies are available FIFO.  This only works at the time you add the book to your RL, as the system will not count down for you.  Depending on the book tho, some lines move faster than others.

Actually, the reminder list does seem to count down.

The Reminder list will only tell you how many books are in the system at any given time.  It will NOT tell you where your book is in the FIFO line.  There's no way to know that.   So the best thing to do is add the book to your Reminder List and make a note of how many copies are in the system on the date you posted the book to your bookshelf.

Also, as Denise said, some books are in more demand than others and they move through the system faster.  If you have the room to store the books, they will ALL eventually float to the top of the FIFO list. 

Plus, if someone uses the "add more" button to order from your shelf, then your book could be number 100, 200, 300 in line and still get ordered as the second or third book someone is ordering from your bookshelf.