'Fifties' in the Bible

for when Jezebel destroyed the prophets of the Lord, Obadiah took a hundred prophets and hid them by fifties in a cave, and provided them with bread and water.)

Has it not been told to my lord [Elijah] what I did when Jezebel killed the prophets of the Lord, how I hid a hundred of the Lord’s prophets by fifties in a cave, and provided them with bread and water?