And yet, there seems to be no shortage of posters available to display at the theaters.
why would there be a shortage for the vast majority of titles?
They print based on perceived need AND quantity needed to get a printing price point.
maybe it costs $5000 to print 20,000 posters, but it might only cost $6000 to print 30,000 posters or $7000 to print 50,000 posters
after your initial order, which includes the cost of 'set-up', increasing print quantities is negligible. Promotions departments will print a determined excess to needs as the worst thing that could happen is to run out.