the Lord appointed seventy-two others: The mission of the twelve disciples represented the ministry of Jesus to Israel (9:1–6). This mission represented his outreach to the Gentiles. The number 72 represents the nations of the world. Genesis 10 lists 70 nations in the Hebrew text, but the Greek Old Testament (the Septuagint) lists 72.
Luke, who used the Septuagint, likely wrote 72, and then a later scribe changed it to match the Hebrew text. The main idea is that the good news about Jesus and the kingdom of God is for both Jews and Gentiles.