The Meaning of Numbers: The Number 21
Twenty-one symbolizes the great wickedness of rebellion and sin. After the children of Israel left Egyptian bondage they had 21 major rebellious events as their traveled and wandered the wilderness.
The number 13, which symbolizes depravity and sinfulness, and 21 are closely related. The meaning of twenty-one in the Bible is an outgrowth of thirteen. Adding thirteen (sinfulness) and eight (symbolizing a new beginning) gives us twenty-one or a number that represents beginning a new level or effort that is sinful and disobedient against God.
On the last day of God's seven-day Spring festival, the 21st of the month of Nisan, the Eternal carried out his last judgment against the Egyptians. He drowned Pharaoh and his entire army in the Red Sea as they were pursuing the Israelites in order to make them slaves again.
The last day of God's annual Fall Feast of Tabernacles, which occurs every year on the 21st day of the seventh Hebrew month, pictures the end of Christ's 1,000 year reign on earth when the devil is loosed out of his prison for a short time. His very first task, along with his demons, is to lead people into war and rebellion against all that is Holy. After a short period of time God executes His final judgment against the Satanic forces by consuming them in fire (Revelation 20:10).
Appearances of the number twenty-one
One of the top ten shortest books in God's word, Obadiah, has only 1 chapter with 21 verses.