Who knows twenty-one?
Please cite/link your sources, if possible. After about one business day, I will:
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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)
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.