Mark 3:13-19 Jesus sends out the Twelve

Jesus went up the mountain and summoned those whom he wanted
and they came to him.
He appointed Twelve, whom he also named Apostles,
that they might be with him
and he might send them forth to preach
and to have authority to drive out demons:
He appointed the Twelve:
Simon, whom he named Peter;
James, son of Zebedee,
and John the brother of James, whom he named Boanerges,
that is, sons of thunder;
Andrew, Philip, Bartholomew,
Matthew, Thomas, James the son of Alphaeus;
Thaddeus, Simon the Cananean,
and Judas Iscariot who betrayed him.

In this passage, we see the beginning of the Church. Jesus appoints Twelve Apostles to minister to the people. Apostles come from the Greek word "To Send". In other words, the Apostles are those Jesus sends out into the world to preach and proclaim the Kingdom of God.  The reason the number twelve is significant is because it connects the actions of the twelve apostles to the twelve tribes of Israel. In other words, it demonstrates God's saving hand in the history of humanity.