Because twelve organises a circle and a year, it became the standard number for a complete set of parts, and traditions reach for it whenever they need one.
Israel. The twelve tribes are the foundational example, and the lists do not agree: Levi is sometimes counted and sometimes replaced by the division of Joseph into Ephraim and Manasseh, so that twelve is preserved by adjusting the membership. That is characteristic. The number is the fixed thing and the contents are made to fit.
Christianity. The twelve apostles are chosen to correspond to the tribes, a correspondence the gospels make explicit. Revelation 21 gives the New Jerusalem twelve gates, twelve foundations and twelve thousand furlongs. The twelve minor prophets are a single scroll in Jewish tradition, counted as one book.
Shi'a Islam. Twelver Shi'ism, the largest branch, takes its name from the twelve imams, the last of whom is held to be in occultation.
Greece. The twelve Olympians are a canonical set whose membership varies between sources, with Hestia and Dionysus exchanged in different lists, again, the number fixed and the contents negotiable. The twelve labours of Heracles were consolidated into that number later than the individual stories, and the metopes of the temple of Zeus at Olympia in the fifth century BCE are among the earliest places the set of twelve is displayed.
Buddhism gives the twelve nidanas, the links of dependent origination.
The pattern is consistent enough to be worth stating as a rule. When a tradition needs a complete and orderly set, it very often chooses twelve, and then edits the membership until it fits. The number comes first.