Twenty-one in the Bible is associated with four major topics and a minor one.  The minor topic is found in 1 Chronicles 24 + 25.  These references are the list of people who have a job that has been assigned to them and the numerical order of these people.

The four major topics are Zedekiah, Daniel’s fasting, the seventh month, and the Passover and unleavened bread.

1. 2 Kings 24:18, 2 Chronicles 36:11, and Jeremiah 52:1 reference that King Zedekiah was twenty-one years old when he began his reign of Jerusalem/Judah.  He would have been ten years old when his father Josiah died.  He was the last king of Israel, until Jesus, and he did not follow God or listen to the prophets.  2 Chronicles 36 mirrors Jeremiah 52 and gives a good summary of his life.  Nebuchadnezzar made him king and changed his name from Mattaniah (gift of God) to Zedekiah (Yahweh is righteousness).  He is one of several people in the Bible to have their name changed.  The change may have been to remind him to follow the rules!  The ten and twenty-one we will see again in another topic. 
2. Daniel 10:13 is part of the story of Daniel’s fast.  The twenty-one days have become important parts of fasting and warfare narratives.  In studying this I feel it is important to remember the reason and the timing of his fast.  The reason was the ending of the seventy years of rest for the land.  Okay, that is also the time that Jerusalem would be vacant before Cyrus king of Persia would let Israel go home.  If “the first month” was the Jewish religious calendar this fast went through the time of Passover.  (Which started on the tenth of the month and went to the twenty-first of the month with the end of Feast of the Unleavened Bread.  So, Daniels’s fast could correspond to the birth/rebirth of the nation of Israel.)
3. Exodus 12:18 is the reference for the first Passover and the ending of the Feast of Unleavened Bread.  This should have been the time period for the crossing of the Red Sea and a time of freedom for Israel.
4. The last major topic also deals with the Feast on the twenty-first day of the seventh month.  The best reference is Haggai 2:1 when a word of encouragement went to Zerubbabel.  This occurred in the seventh month or during the time of the Feast of Tabernacles.  This commemorates Israel living in tents for forty years.  This feast comes after the Day of Atonement, which is on the tenth day of the seventh month. (Study Leviticus 23.) The prophet Jeremiah was also being taken to Egypt, against his will, during this time period after the fall of Jerusalem.

The twenty-first letter of the Jewish alphabet is “shin” and it is the letter that looks like a crown.  The number value of shin in the Hebrew language is 300.  This letter is associated with the Day of Atonement (Yom Kippur) because its gematria is 300. To appreciate the full scope of shin go to Chabad.org as they have a good article on this topic.

An interesting “twenty-one” I found is in Luke 3:34-38.  Start with Adam, as number one, and count to Abraham.  I just read in my devotional book that we give up the curse of Adam at the cross and take on the blessing of Abraham – who is twenty-one from Adam.

 

https://www.chabad.org/library/article_cdo/aid/137093/jewish/Shin-Sin.htm

https://www.hebrew4christians.com/Grammar/Unit_One/Aleph-Bet/Shin/shin.html

https://imgbin.com/png/b8eFPpq5/shin-hebrew-alphabet-letter-png

I do not believe in “good luck” numbers as Jesus does not do things by luck.  But because of this study, I would associate the number twenty-one with the Feasts of Israel and the beginning of the nation.