The Salt Lake Temple Pinnacles
Each of the six spires on the Salt Lake Temple is adorned with twelve pinnacles. One pinnacle is set on each corner of the three-levels on the spire. The symbolic significance of the pinnacles builds on the symbolism of the towers and spires, which pertains to the holy priesthood. The East tower and spired represents the Melchizedek Priesthood and the West tower and spired represents the Aaronic Priesthood. The pinnacles on the Melchizedek tower spires represent the Twelve Apostles. The pinnacles on the Aaronic tower spires represent the Ward and Stake High Council.
The number 12 itself is also rich in Judeo-Christian symbolism. In the Old Testament, there were twelve tribes of Israel, the altars of the temples measured twelve cubits, and various items in multiples of twelve were placed thereon. Also, there were twelve oxen holding up the brazen sea at the temple. In the New Testament, Jesus called twelve apostles, and in the Book of Revelations it says that the Holy City of God in Heaven has twelve gates.
The number twelve is also a representation of the Holy Priesthood and multiple uses of twelve symbolizes a fullness of that priesthood. Furthermore, the number twelve symbolizes a union of the temporal with the spiritual, or a joining of Heaven and Earth. These semiotics are quite fitting for the temple since Mormons view it as a sacred space where Heaven and Earth meet. The covenants made and the blessings pronounced inside the temple, under authority of the priesthood, are valid on Earth and in Heaven. For example, temple marriages are solemnized temporally and for eternity.
The priesthood significance of the number 12 also explains why Jesus Christ chose twelve men, instead of six, eight or even twenty, to be Apostles. It wasn't an arbitrary number. The Priesthood Authority of the Apostles gives them the keys for the fullness of the priesthood and the power to bind on Earth and in Heaven. (Matthew 16:19)