Christianity adopted the importance of the number 12 from Jewish scriptures (where kings, judges and leaders often come in twelves), from Babylonian culture that infused the mythos of the region, and from pagan religions of the Roman empire, who often divided the heavenly realm into 12 areas, each ruled by a different god. Also, pagan religions such as Mithraism had the son of God followed by 12 disciples. So when the stories of Jesus emerged, it was only fitting that he too had 12 disciples. Because these stories were not based on 12 actual people there are massive contradictions in the New Testament about who the 12 were - every list is different. See Mark 3:16-19, Matt. 10:2-4, Luke 6:14-16, John 21:2 and Acts 1:13. And although the 5th book of the New Testament is called "The Acts of the Apostles", their actual deeds are very brief, and some disciples are not mentioned at all. It is best to consider any "multiple of 12" element in the Bible to be based on copied pagan myths rather than being a historical account.
Full page: The 12 Disciples: The Christian Take on Ancient Astrology