The number 7 occurs many times in the Bible, beginning with the seven days on which God created and rested (Gen 2:3) through to the seven angels who had the seven bowls full of the seven last plagues (Revelation 21:9). It is used so often that God must be telling us something by its repetition, although he never gives an explicit explanation.
Most Bible readers come to the conclusion that the number seven represents completion or perfection. The creation was completed on the seventh day, for example, and the seven plagues of Revelation represent the completed punishment of the wicked.
Further reading: Davis, J.J. Biblical numerology, Baker Book House, 1968.