The number twenty-one is the number that follows twenty. In the world it could represent gambling (blackjack) or becoming legal to drink alcohol in the United States.
Positive: Dreaming of the number twenty-one could represent perfection through excellence or the representation of God and the Temple.
Negative: The number twenty-one could also indicate the overwhelming nature of sin or sins that are offensive to God.
(2 Timothy 3:2-5).