Luke is the only Gospel writer to mention a special sending out of seventy (-two) disciples by Jesus (Luke 10:1). The number of disciples is uncertain; the manuscript evidence is divided between reading ‘seventy’ and ‘seventy-two.’ The MT (Masoretic Text) numbers seventy nations, but the LXX (Septuagint) has seventy-two nations (Luke is following LXX?). The significance of the number has been traced to the number of the Sanhedrin or the number of elders in Israel (Exodus 24:1), but the most likely explanation is that Jesus is here reflecting on Genesis 10 with its listing of the seventy known nations of the then known world. Taken in this way, the number signifies that Jesus is sending his representatives into all the known nations of their day. The world he created he must also redeem.
Victor Hamilton, The Book of Genesis, p.348.