And he went up on the mountain and called to him those whom he desired, and they came to him. And he appointed twelve (whom he also named apostles) so that they might be with him and he might send them out to preach and have authority to cast out demons. He appointed the twelve: Simon (to whom he gave the name Peter); James the son of Zebedee and John the brother of James (to whom he gave the name Boanerges, that is, Sons of Thunder);
Gospel of Mark 3:13-17 ESV
Jesus calls 12 followers of him to be his disciples (or also called apostles). Their task is to be with Jesus and to be sent out to preach and cast out demons. On top of that their number twelve is also similar with the number of the twelve tribes that represented God’s people in the Old testament.