Thermopylae wasn't fought by 300 Spartans alone.
In 480 BC, a Greek army of about 7,000 held the pass of Thermopylae for three days against a far larger Persian force. Only when a traitor showed the Persians a mountain path did King Leonidas send most away. Around 300 Spartans, 700 Thespians and 400 Thebans stayed. The Thespians died just the same, but rarely appear in the legend.
Useful: When a story has only one hero, the rest of the cast is usually missing.
Source: en.wikipedia.org