Why 12?

Twelve is the number that marks a complete cycle. There are 12 hours on the clock, 12 months in the calendar year, 12 inches in one foot, 12 items in a dozen.   In modern numerology, 12 is the number associated with knowledge, wisdom, and oracle-like serenity. Astrologers believe that the 12th house of the zodiac rules the most creative and magical parts of our psyche.

The mystics of yore and ancient religions 

had a thing for 12 too.
Think about it; there are:

12 signs of the zodiac

12 Apostles

12 Tribes of Israel

12 Fruits on the Tree of Life