The 13 persons mentioned were probably the chief priests of the course (shift) which was at the time performing the temple service.
Related Commentaries of Nehemiah 8:4
Nehemiah 8:4
4 And Ezra the scribe stood upon a pulpita of wood, which they had made for the purpose; and beside him stood Mattithiah, and Shema, and Anaiah, and Urijah, and Hilkiah, and Maaseiah, on his right hand; and on his left hand, Pedaiah, and Mishael, and Malchiah, and Hashum, and Hashbadana, Zechariah, and Meshullam.