In short
Jesus specifically chose twelve apostles to represent the twelve tribes, as the new Israel, the new people of God. The number twelve is also the product of the divine number (three) multiplied by the number representing the world (four).
The number of apostles called by Jesus stands in direct relation to the number of the tribes of Israel (see Matthew 19:28). The disciples represent the new Israel, the new people of God. The twelve apostles were Jesus’ first disciples. Before they could become missionaries, however, they first had to be students.1
The number of the apostles, twelve, not only represents the twelve tribes of Israel, but is also the product of the divine number (three) multiplied by the number representing the world (four).2
1 And he called to him his twelve disciples and gave them authority over unclean spirits, to cast them out, and to heal every disease and every affliction.