Upvote:3
There were a lot more than 300 Greeks at the Battle of Thermopylae. The 300 were just the Spartan contingent. According to Herodotus the whole Greek army had about 5,000 men in it from all parts of Greece.
Upvote:10
The Greek historian Herodotus is the main primary source of information about the battle of Thermopylae. Most other records of the battle come from historians who lived centuries after the battle. They are all fairly consistent with each other.
How did Herodotus get his information? The most common way historians did for centuries. By travelling the world and speaking to natives of the places he visited. How reliable is he? that is a question that has been asked since the time of Herodotus's death.
Here is what Herodotus says about his works:
I am bound to tell what I am told, but not in every case to believe it.
Book 7, Ch. 152.
Unfortunately, if there where any Persian records on Thermopylae, none have survived to the present (or been found). We have to rely mainly on the Greek historians on this matter.