Yes, it’s true that there were over a thousand Greeks at Thermopylae. The original force was even larger, over 5,000 Greeks (according to Wikipedia). The 300 Spartans who stayed after Xerxes found the goat path and the Greek army retreated were only part of the forces left behind, which included a whole bunch of Spartan serfs and some other Greeks. Thus, it wasn’t only 300 Spartans who died there, but almost 2,000 Greeks total.

On the other hand, I can’t really fault the filmmakers or Frank Miller for leaving them out. The battle helped rally the Greek City-States, and the legend among the Greeks very quickly became one of “300 Spartans.” After all, it’s much more impressive to think that only 300 men could hold an army as large as Xerxes’ at bay. In this sense, the glorification of the Spartan sacrifice is very much in tune with the mythology the battle bred.