The number six in the Bible represents man and rebellion. Both man and serpent were created on the sixth day, which is why the number represents both man and the evil that weakens him.
In the Greek alphabet, the number six is represented by a symbol called the "stigma" versus an actual number. Revelation 13:18 displays the number of the beast using the Greek symbols for 600, 60 and 6.
There are other mentions of six within the Bible, such as Exodus 31:15 where it states man is commanded to labor for six days. The Ten Commandments are listed in Exodus 20:13, with the sixth commandment reading "thou shall not kill."
In Matthew 6:13 displays the sixth clause in the prayer "Our Lord's Prayer" that petitions that man not be led into sin and be delivered from evil, which is symbolic of the meaning of the number six.
Other interesting facts about the number six in the Bible include, Jesus suffered on the cross for six hours, the world turned dark on the sixth hour in Hebrew time when Christ was placed on the cross, and all six letters that represent the system of the Roman Empire yield the number 666 when added together.