I'm not sure of any numbers, but I do know that towards the end of the catalog sales period they cut production of the retiring stamps a bit so they can get as close as possible to the number of stamps needed. Towards mid- to late- June retiring sets are often on backorder for a day while another run of stamps is produced. Of course, all orders submitted by demos by June 30th will be honored-- no "running out" of stamps!
Be sure to contact your demo to find out when she needs your order to be able to submit it to SU.
As Stephanie said, many retired stamp sets are donated either through demo requests to schools and other charitable causes or to those causes directly (such as the RMHC).