'Thirty' in the Bible

This is the list of David’s mighty men: Jashobeam, the son of a Hachmonite, the chief of the thirty [heroes]. He lifted up his spear against three hundred whom he killed at one time.

Three of the thirty chief men went down to the rock to David, into the cave of Adullam, while the army of the Philistines was camped in the Valley of Rephaim.

He was honored among the thirty, but he did not attain to [the rank of] the [first] three. David appointed him over his bodyguard.

Adina the son of Shiza the Reubenite, a chief of the Reubenites, and thirty [heroes] with him,

Ishmaiah of Gibeon, a mighty man among the thirty, and [a leader] over them; Jeremiah, Jahaziel, Johanan, Jozabad of Gederah,

The Levites thirty years old and upward were counted, and their number man by man was 38,000.

This is the Benaiah who was the mighty man of the thirty and was in charge of the thirty; and Ammizabad his son was over his division.