The city of Tegea contributed 500 hoplites to the Greek force at Thermopylae. Herodotus lists them alongside 500 hoplites from Mantinea, making 1,000 soldiers from those two Arcadian communities.
Tegea was an important city in Arcadia in the central Peloponnese. Its troops joined the coalition assembled to resist Xerxes I’s Persian invasion in 480 BC. They served among the Peloponnesian contingents commanded within the broader Greek army led by the Spartan king Leonidas.
The famous story of Thermopylae often focuses exclusively on the 300 Spartan citizens, but the defenders included thousands of allies. Herodotus’s list includes Tegeans, Mantineans, other Arcadians, Corinthians, Phliasians, Mycenaeans, Thespians, Thebans, Phocians, and Locrians. The Tegean figure is sometimes confused with the combined total of Tegeans and Mantineans, which was 1,000. The question asks specifically about Tegea, so 500 is the single canonical answer.