The parallel accounts in Matthew 10:5-15 and Mark 6:7-13 indicate that only the twelve disciples were sent on this mission with no mention of the "other seventy." It was possibly a different mission indicating that, by this time, Jesus had many disciples following Him in addition to the twelve; He was concerned with training and using them as well. Possibly He selected seventy of them as symbolic of the "seventy elders" of Israel (Exodus 24:1), just as the twelve apostles similarly represented the twelve tribes.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8