The numbers were 10,000 vs. anywhere between 120,000 and 2 million. 6,000 of the 10,000 didn't show up until the fighting had started. All 300 Spartans and all 700 volunteers died, along with 80,000 Persians, at which point the Persians gave up. Frank Miller chose to emphasize the Spartans, who quite obviously killed a large chunk of those Persians. He also set the Persian army at 1 million men, half of their highest enlisted total, while still romanticizing the whole underdog thing to a great degree. For an 80:1 kill/death ratio, changing a few numbers doesn't really seem to matter. Those corpses must've reached Conan the Barbarian-esque proportions.