This small number suggests careful selection, inasmuch as some of the larger tribes could easily have supplied a much greater number.
All the tribes.
Some think this may have included the Levites, not perhaps to bear arms, but as a service unit behind the front line. But the fact that the Levites received their share of the booty from that portion of it assigned to those who did not go to war (vs. 30, 37-41) seems to preclude such an idea.