Why not assign 12 patron Buddhist deities instead of eight?
The most plausible reason involves Chinese Taoism, Feng Shui 風水 (geomancy), and divination, for there are eight interrelated symbols (or trigrams) in Chinese cosmology called the Bā Guà (Chn. = 八卦, Jp. = Hakka). Each consists of three lines and each is associated with either the yin or yang principle. The eight trigrams symbolize the unceasingly changing balance of forces and are used in divination. See Divination section for details. The origin of this grouping of eight is unknown to me, but it likely originated in China. The grouping was popular at least by the Edo period (1603 – 1867) for it was listed in the Butsuzō-zu-i 仏像図彙 or “Collected Illustrations of Buddhist Images,” one of Japan’s major studies of Buddhist iconography (published in 1690 AD). Each of the eight Buddhist protectors is linked to one of the 12 zodiac animals, to one of the eight directions and semi-directions, and to one of the 10 calendar signs. The list goes on, with these Buddhist protectors also linked to th
