What is the importance of the number 12 in Islam?

Question

The number 12 is frequently seen in nature and in Quran.
What is the reason for this?

Examples:

وَإِذِ اسْتَسْقَى مُوسَى لِقَوْمِهِ فَقُلْنَا اضْرِب بِّعَصَاكَ الْحَجَرَ فَانفَجَرَتْ مِنْهُ اثْنَتَا عَشْرَةَ عَيْناً قَدْ عَلِمَ كُلُّ أُنَاسٍ مَّشْرَبَهُمْ كُلُواْ وَاشْرَبُواْ مِن رِّزْقِ اللَّهِ وَلاَ تَعْثَوْاْ فِي الأَرْضِ مُفْسِدِينَ
And [recall] when Moses prayed for water for his people, so We said, "Strike with your staff the stone." And there gushed forth from it twelve springs, and every people knew its watering place. "Eat and drink from the provision of Allah , and do not commit abuse on the earth, spreading corruption."
Bakara 60 (2/60)

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إِذْ قَالَ يُوسُفُ لِأَبِيهِ يَا أَبتِ إِنِّي رَأَيْتُ أَحَدَ عَشَرَ كَوْكَبًا وَالشَّمْسَ وَالْقَمَرَ رَأَيْتُهُمْ لِي سَاجِدِينَ
[Of these stories mention] when Joseph said to his father, "O my father, indeed I have seen [in a dream] eleven stars and the sun and the moon; I saw them prostrating to me."
Yusuf 4 (12/4)
(11 brothers + their little brother Yusuf = 12 brothers at total)

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وَلَقَدْ أَخَذَ اللّهُ مِيثَاقَ بَنِي إِسْرَآئِيلَ وَبَعَثْنَا مِنهُمُ اثْنَيْ عَشَرَ نَقِيبًا وَقَالَ اللّهُ إِنِّي مَعَكُمْ لَئِنْ أَقَمْتُمُ الصَّلاَةَ وَآتَيْتُمُ الزَّكَاةَ وَآمَنتُم بِرُسُلِي وَعَزَّرْتُمُوهُمْ وَأَقْرَضْتُمُ اللّهَ قَرْضًا حَسَنًا لَّأُكَفِّرَنَّ عَنكُمْ سَيِّئَاتِكُمْ وَلأُدْخِلَنَّكُمْ جَنَّاتٍ تَجْرِي مِن تَحْتِهَا الأَنْهَارُ فَمَن كَفَرَ بَعْدَ ذَلِكَ مِنكُمْ فَقَدْ ضَلَّ سَوَاء السَّبِيلِ
And Allah had already taken a covenant from the Children of Israel, and We delegated from among them twelve leaders. And Allah said, "I am with you. If you establish prayer and give zakah and believe in My messengers and support them and loan Allah a goodly loan, I will surely remove from you your misdeeds and admit you to gardens beneath which rivers flow. But whoever of you disbelieves after that has certainly strayed from the soundness of the way."
Maide 12 (5/12)

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إِنَّ عِدَّةَ الشُّهُورِ عِندَ اللّهِ اثْنَا عَشَرَ شَهْرًا فِي كِتَابِ اللّهِ يَوْمَ خَلَقَ السَّمَاوَات وَالأَرْضَ مِنْهَا أَرْبَعَةٌ حُرُمٌ ذَلِكَ الدِّينُ الْقَيِّمُ فَلاَ تَظْلِمُواْ فِيهِنَّ أَنفُسَكُمْ وَقَاتِلُواْ الْمُشْرِكِينَ كَآفَّةً كَمَا يُقَاتِلُونَكُمْ كَآفَّةً وَاعْلَمُواْ أَنَّ اللّهَ مَعَ الْمُتَّقِينَ
Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred. That is the correct religion, so do not wrong yourselves during them. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him].
Tawba 36 (9/36)

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• Isa (Christ) had 12 hawaris.
• There are 2 x 12 = 24 hours in a day. 12 for day and 12 for night.
• There are 12 months in a year.
• Buddha had 12 students. (Buddha could be an ancient prophet.)
• There are 12 kind of years in Chinese calender.
• Some mythological stories involve with the number 12. (Considering that mythology is corruption of ancient ummats makes sense).

It looks like that the number 12 has something to do with periodic movement and circulations.

What makes this number so special in Quran and in nature?

Answer 1

About ancient civilizations and the number 6

In the ancient times, mankind was seeing the astrology as a science. They were studying it very deeply and the order in the sky was tried to replicate in the earth. Probably because of this, ancient civilizations used 6 as a base in their mathematics (today we use 10 because it is easy to calculate). The division of time was based on number 6 in many societies like Chinese, Central Asian societies like Turks and Mongols, Egypt etc. All used a calendar based on 6. They also divided the day into multiple of 6, designed clocks according to that. They divided the year into multiple of 6 etc.

About the 12 number of Children of Israel

I think the Children of Israel may divided into 12 tribes because Prophet Josef(puh) has 11 brothers. They probably were influenced by this ancient tradition, thus tried to replicate it somehow. Probably the main reason for why and how they corrupted their belief was this tradition. Although I am not sure.

About the 12 springs and 12 leaders, it is about their division which was already there. I heard that these 12 tribes of Children of Israel were constantly fighting and struggling each other back in that time, so Allah sent them not 1, but 12 springs. There maybe another reason for that, I don't know. Also if there were 12 tribes, it is natural that there were 12 leaders.

About 12 months in the eyes of Allah

In the Turkish tafseer of The Presidency of Religious Affairs of Turkey, it is explained as follows (I am summarizing and translating):

Back in the Era of Ignorance, the Arabs were considering 4 months as haram, as a belief coming from Prophet Abraham(puh). In these months, they were trying to avoid fights, wars etc. They were zilkade, zilhicce, muharrem and receb. However, some tribes were counting 8 months as haram, while some tribes were not believing any haram months. Because of this, the other tribes had to be careful about these tribes, because they could start a war in these months. The tribes back then were always fighting each other, also zilkade, zilhicce and muharrem are consequtive months, so it was hard to follow this belief.

Because a moon year is 11 days shorter than a sun year, the time of pilgrimage was changing all the time (as today). They also didn't want pilgrimage which is in zilhicce (the last month) to be disturbed because of improper weather conditions for trade. They want that the pilgrimage should be in a time appropriate for trade. In order to do that, they added 1 week for each 6 months, thus gained 1 month in every 2 years and added this month to the end of the year. This means that, in every 2 years, a year was 13 months and the consecutive 3 haram months were no longer consecutive.

They also counted some haram months like receb and muharrem as not haram when they needed to fight and counted the months safer and şaban as haram for that year.

As a conclusion, with the verse Surat At-Tawbah 9/36, all of these wrong actions and attitudes were corrected as in the time of Abraham(puh).

About you list

For 12 havaris of Jesus(puh), I don't know if there were exactly 12 of them. I searched in the Quran and couldn't find any number of hawaris. This number may be determined in later times with the corruption based of old pagan belief.

For the division of years, months, days etc, I tried to explain this.

I have not enough information about Buddha.

As we learn from the verse Surat At-Tawbah 9/36, there is an order in the universe based on 6 or 12. So, my idea is that this order was discovered by previous civilizations and the number entered the ancient beliefs and myths. And later on, it is entered religions such as Christianity, Judaism etc as a corruption and its real meaning is forgotten.

Answer 2

The number 12 gets repeated because the children of Israel had 12 tribes and because Qur'an deals in detail about the children of Israel. (Your first and third reference are from the events of children of Israel)

Your second reference (from the life of Joseph, on whom be peace) - the number can also be counted as 13 (11 stars, a sun and a moon) or as fourteen (11 brothers + Joseph + Parents)

To conclude. There is as such no (apparent) importance. There are several other numbers used in Qur'an as well - for example:

 Qur'an 74:30:: Over it are Nineteen. 
 Qur'an 18:25:: So they stayed in their Cave three hundred years, and add nine