According to The Bible Study Site, the number 21 symbolizes sin and rebellion. This is because once the children of Israel left Egyptian bondage, there were 21 different rebellious events that took place.
The Bible Study Site says the numbers 13 and 21 are closely related. This is because 13 symbolizes sinfulness and depravity. In fact, the meaning of 21 in the Bible is an outgrowth of the number 13. When adding 13, which is sinfulness, and the number 8, which represents a new start, the sum is 21. This is why it is said the number 21 represents a new level of sin or a new effort to be sinful.