Who knows twenty-one?
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Twenty-one are the books of Tanach that use the "normal" system of trop (cantillation).
(Actually, 21 and a fraction, since the first 36 and the last 11 verses of Iyov - the framing narrative - use it too.)
Twenty-one are the days when we recite the complete Hallel (outside of Israel): the first two days of Pesach (and according to some customs, also the first two nights), two days of Shavuos, nine of Sukkos/Shemini Atzeres/Simchas Torah, and eight of Chanukah. (Erachin 10a)
21 are the days between the seventeenth of Tammuz and the ninth of Av.
There are twenty-one sins for which one can get kares or lashes, but not court-imposed death. Source: Rambam, Sanhedrin 19.
The letters engraved into the luchot cut through from one side to the other. The holes in the center of the letters Mem and Samech (ם & ס) were held up by a miracle. This occurs 21 times on each side of the luchot (& only in the first five commandments). (Parshat Va'etchanan)
Twenty-one is the Gematria of "ach", which serves to diminish. When it says the Flood left "ach Noach", the diminution combined with the onomatopoeia indicate that Noah was retching blood. (Yuck!)
Twenty-one is the age below which the Rambam recommend medically against drinking alcohol.
(Better late than never?)
21 are the days of dealing with tzara'as (in most cases).
21 is the number of times the word ברית or some variation thereof appears in sefer Tehillim (You can count yourself here, make sure not to count repeat results)
21 are the verses in Ovadiah.
21 is the public drinking age (in the U.S. http://www.law.cornell.edu/uscode/23/158.html ) and this is enforced by all states with many making exceptions for under 21 drinking in private settings, small groups, parental permission and for sacramental (kiddush, havadalah, bris milah, etc.) purposes.