Who knows eighteen?
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Eighteen are the years before one gets hitched. (Ben Shemonah Esrei LiChupah: Avos 5:21)
Shemona Asar was the number of berachot in the Shemona Esrei, before the addition of Birkas HaMinim." (Berachot 28b)
Eighteen is the value (in maneh) of the linen clothing that the Kohen Gadol wore for his first entry into the Holy of Holies each Yom Kippur, according to the majority opinion (Yoma 34b).
There are eighteen sins for which the punishment is misa bide shamayim but for which one can get lashes. Source: Rambam, Sanhedrin 19.
18 are the instances of prayer in the Torah, as enumerated by Rav Saadya Gaon in the introduction to his Siddur: Gen 20:17, 25:21, 32:10, Exo 8:8, 8:26, 9:33, 10:18, 15:1, 32:11, 33:13, 34:9, Num 11:2, 12:13, 14:13, 21:7, 21:17, 27:16, Deu 3:23.
He also mentions that there are 18 days the Torah sanctified as Moadim: 7 for Pesach, 1 each for Shavuot, Rosh haShana, and Yom Kippurim, and 8 for Sukkot.