# 12 has historically been fascinating number for hunman beings ,and early civilizations often gave prominance to this number i.e Time ( 12 months a year,12*2 hours a day), measurement and weights , math , astronomy and all other subjects that preoccupied early humans.
Religions ,being man made philosophies were no exempt and #12 not only played a role in semetic religions but subcontinent India and Chinese religions.
I have no knowledge why shite settled for 12 emams ( perhaps they had the foresight of knowing that there would be gosale-gan who would worship 11,000 offsprings of 11 and that is good enough).Perhaps 12 being a magical composite number was a good number of the chosen absent one...I don't know ...
I suspect that fascination with #12 , may have been rooted in astrology in older civilizations and gradually worked their way into man made semitic religions..
Particularly Mithraism>>>>>Christanity>>Shite Islam
Below is about significance of #12 in religion. Doesn't answer your question but interesting:
In religion and mythology
There are twelve ' Jyotirlingas' (epitome of God Shiva) in Hindu Shaivism. The shaivites (orthodox devotees of God Shiva) treat them with great respect and these are visited by almost every pious Hindu at least once in a lifetime. The number 12 is very important in many religions, mainly Judaism, Christianity, and Islam, and also found in some older religions and belief systems.
In Antiquity there are numerous magical/religious uses of twelves.[2] Ancient Greek religion, the Twelve Olympians were the principal gods of the pantheon. The chief Norse god, Odin, had 12 sons. Several sets of twelve cities are identified in history as a dodecapolis, the most familiar being the Etruscan League. In the King Arthur Legend, Arthur is said to have subdued 12 rebel princes and to have won 12 great battles against Saxon invaders. [source: Benet's Reader's Encyclopedia, 3d ed]
The importance of 12 in Judaism and Christianity can be found in the Bible. The biblical Jacob had 12 sons, who were the progenitors of the Twelve Tribes of Israel, while the New Testament describes twelve apostles of Jesus; when Judas Iscariot was disgraced, a meeting was held (Acts) to add Matthias to complete the number twelve once more. (Today, The Church of Jesus Christ of Latter-day Saints has a Quorum of the Twelve Apostles.)
The Book of Jude contains much numerical symbolism, and a lot of the numbers mentioned have 12 as a divisor. 12:1 mentions a woman — interpreted as the people of Israel, the Church or the Virgin Mary — wearing a crown of twelve stars (representing each of the twelve tribes of Israel). Furthermore, there are 12,000 people sealed from each of the twelve tribes of Israel, making a total of 144,000 (which is the square of 12 multiplied by a thousand).
In Orthodox Judaism, 12 signifies the age a girl matures (bat mitzvah)
There are 12 days of Christmas. The song Twelve Days of Christmas came from the traditional practice of extending Yuletide celebrations over the twelve days from Christmas day to the eve of Epiphany; the period of thirteen days including Epiphany is sometimes known as Christmastide. Thus Twelfth Night is another name for the twelfth day of Christmas or January 5 (the eve of Epiphany). Similarly, Eastern Orthodoxy observes 12 Great Feasts.
In Twelver Shi'a Islam, there are twelve Imams, legitimate successors of the prophet Muhammad. These twelve early leaders of Islam are—Ali, Hasan, Husayn, and nine of Husayn's descendants.
Imāmah (Arabic: إمامة) is the Shī‘ah doctrine of religious, spiritual and political leadership of the Ummah. The Shī‘ah believe that the A'immah ("Imams") are the true Caliphs or rightful successors of Muḥammad, and Twelver and Ismā‘īlī Shī‘ah further that Imams are possessed of supernatural knowledge, authority, and infallibility (‘Iṣmah) as well as being part of the Ahl al-Bayt, the family of Muhammad.[1] Both beliefs distinguish the Shī‘ah from Sunnis.
In Hinduism, the sun god Surya has 12 names. Also, there are 12 Petals in Anahata (Heart Chakra.)
The Twelve Labours of Hercules