Why did they only have 300 soldiers?

If you are talking about the Spartans at Thermopylai, they had 300 hoplites (heavy infantry), 2,100 light infantry, and there were about 7,000 from other Greek cities.

The Saprtan 300 heavy infantry were the personal bodyguard of King Leonidas. The rest of the Spartan army was kept at home to protect against the threat of an amphibious attack by the Persian navy.