Friends, today's Gospel recounts Jesus selecting and appointing the Apostles. Bible scholar and theologian N.T. Wright has explained why Jesus commissioned twelve disciples as Apostles.
Wright tells us that when a first-century Jew spoke of the arrival of God's kingdom, he was taken to mean something very specific. He was announcing that the Temple was going to be restored, that the proper worship of Yahweh would be obtained, that the enemies of Israel would be dealt with and that, above all, the tribes of the Lord would be gathered and, through them, the tribes of the world.
Recall the great vision from the second chapter of Isaiah: "The mountain of the Lord's house shall be established as the highest mountain.… All nations shall stream toward it." This is why Jesus chose twelve disciples, evocative of the twelve tribes. They would be the prototype and the catalyst for the gathering of Israel and hence the gathering of everyone. They would be the fundamental community and sign of unity.