The Talmud tells us that the choshen (breastplate) of the Kohen Gadol (High Priest) had the names of the 12 tribes along with the names of the three patriarchs (Avraham, Yitzchak and Yaakov). It also had the words Shivtei Yeshurun (“Tribes of the Upright”). It is taught that these 72 letters were divided evenly among the twelve stones on the choshen [The Baal HaTurim on Shemos 28:17]. How many letters were on each of the stones?
72 / 12 = 6 letters on each.