300 Spartans, but 7,000 Greeks.
While there were 300 Spartans at the pass at Thermopylae, there were 7,000 Greeks in total. The film drastically underrepresents Greek numbers in order to make the stand seem more heroic than it was.
I'm not sure if I should submit this as a historical goof. I don't recall if the movie ever said how many Greeks were actually present. But the film is intentionally misleading on this point.
One the third day, when they were outflanked by the enemy, the Spartans dismissed their allies and prepared for a last stand. 700 Thespians insisted on staying with them and died along with the Spartans. Since they made a larger sacrifice, at least in terms of numbers, perhaps this film really should be called "700 Thespians."
The film doesn't ask this, but what's so heroic about dying pointlessly anyway? Sure it took lots of courage, and it's in keeping with the romantic Spartan warrior ethic, but for those of us who have an entirely unromantic and unsentimental view of war, the idea of men queueing up to die when they don't have to and when the battle is already lost strikes me as idiotic, not heroic.