The Jewish people thought of seven as a sacred number. It is used often in the Bible to show fullness, completion, and perfection (Gen 2:2-3; Dan 9:25).
The apostle John used the number seven throughout the book of Revelation: seven churches (1:4), seven stars (2:1), seven lamps (4:5), seven seals (5:1), seven angels with seven trumpets (8:2), seven thunders (10:4), a beast with seven heads (13:1), seven vials or bowls (17:1), and seven kings (17:10).
By using this number, John declared that God will bring about His perfect judgment in the end times. All believers will participate in the full and complete victory of Christ over the forces of evil.
REVELATION 15:1 – And I [the apostle John] saw another sign in heaven. . .seven angels having the seven last plagues.