At the end of most major American films, there's a number. I believe current films are running in the high 40000s. From a little bit of research, when the MPAA rates a film, they give it a unique number.
I've found an old list that seems to go up to 2010, but I'm wondering if there's an official list online somewhere that shows every number. I've been wondering for a while now if there'd be any friendly rivalry to grab number 50,000, which must be out soon ;)