A few against many.
The fight/action scenes are why you see this film. Yes the story of the few against the many is a good one, just don't expect it to be 100% accurate factually.
A brief history lesson then, the Battle of Thermopylae which took place around 480 BC was between an alliance of various Greek states and the invading Persian army.
The famous battle took place in the mountains where the numbers of the Persian army became less advantages. In the end however the Greeks were always going to loose. This was always going to be a "last stand".
They might not have lost after three days if one of their own didn't betray them and show the Persians a way to outflank the Greeks via a locally known path. The defending forces were closer to 1,400 against the 100,000 Persians, yes there were only 300 Spartans in the defending force but they weren't on their own.
This is everything you would expect from Hollywood, so enjoy the action, don't think it is a documentary and historically accurate.