Who knows twenty?
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20 is the age when one can sell inherited real estate. (Hoshen Mishpat 235:9)
Twenty is the age of the Draft. (Numbers 1:3)
20 represents כתר, the highest of the Sefiros, since (a) the initial letter of כתר is כ, which equals 20, and (b) the numerical value of the word עשרים is 620, which equals that of כתר. (Likkutei Torah, Shir Hashirim 35c)
Twenty (point one) minutes are a "hiluch mil" (a Halachic time unit) according to Rabbi Ari Storch's reading of Tosfos.
(The common opinions are 18, 22.5 or 24 minutes instead.)
20 are the years which Hashem waits for someone to get married before deciding that his bones should rot (Kiddushin 29b).
Tosafot Yom Tov on Pirkei Avot 5:21
AT TWENTY, CHASING. The Rav’s second explanation is that the heavenly court only begins to punish a person at 20. This is taken from Bemidbar Rabbah 18:4. It seems this was based on the fact that only those who were twenty years old at the incident of the spies died in the desert (Numbers 14:29).
The maximum height of a Chanukah menorah is 20 amot.
20- As compiled and listed by this Hebrew Wikipedia entry, 20 is the number of instances in Shas where the principle of "chiddush"is invoked.
1) חיוב קרבן בשבועת ביטוי - an “oath of expression” about a future or past event-- ie NOT an oath of testimony (see Shevuot 26b with Tosfos)
2) בשר בחלב - meat and milk (see Pesachim 44b)
3) גיעולי עובדי כוכבים - purging the vessels of non-Jews with boiling water (see Pesachim 44b)
4) קנס - paying a penalty (see Ketubot 38b)
5) נזיר - accepting a partial nazirite vow upon oneself (see Nazir 3b-4a)
6) שפחה כנענית לעבד עברי - Canaanite maidservant to a Jewish slave (see Kiddushin 21b with Rashi)
7) אשת יפת תואר - a beautiful woman who was taken captive in war (inferred from Kiddushin 21b)
8) עדים זוממים - conspiring witnesses (see Sanhedrin 27a)
9) ריבית - charging interest (see Bava Metzia 61a)
10) טומאת שרץ - if a sheretz (creepy crawly creature) imparts "flavor" to a mixture it is still prohibited (see Avodah Zarah 68b)
11) טומאת שכבת זרע - impurity of an emission that's imparted into a mixture is still prohibited (Avodah Zarah 68b)
12) זרוע של איל נזיר - the foreleg of the ram offering that a nazirite must bring (see Chullin 98b)
13) חלב בהמה טהורה - permissibility of consuming milk from a kosher animal (see Bechoros 6a-b)
14) צרעת הבתים - leprosy on houses (see Moed Kattan 7b-8a)
15) יבמה - the widow of a childless man who has brothers (see Yevamos 17b)
16) נבלת עוף טהור - the carcass of a pure bird (see Zevachim 70a)
17) מנחת העומר - the Omer offering (inferred from Menachos 5a-b)
18) טומאת בעלי קרי בהר סיני - the impurity of a person who had an emission by Har Sinai (see Niddah 42a)
19) מזיד בשבועת ביטוי - purposely taking a shevuas bitui (see Shevuot 37a)
20) מוציא שם רע - a man who claims his wife committed adultery (see Ketubot 45a)
(for an explanation of the what the "chiddush" is for each entry on this list , please refer to this answer)
זֶה֩ עֶשְׂרִ֨ים שָׁנָ֤ה אָנֹכִי֙ עִמָּ֔ךְ רְחֵלֶ֥יךָ וְעִזֶּ֖יךָ לֹ֣א שִׁכֵּ֑לוּ וְאֵילֵ֥י צֹאנְךָ֖ לֹ֥א אָכָֽלְתִּי׃ “These twenty years I have spent in your service, your ewes and she-goats never miscarried, nor did I feast on rams from your flock." - Bereshis 31:38-40
Yaakov spent 20 years in Laban's house and did not sleep. (Bereshis Rabbah 68:11)