The number six is typically associated with the creation of man (which occurs on the sixth day of creation), while seven indicates completeness or perfection because it is associated with the creation week. The number 10 is associated with the giving of the law, states About.com.
The number 12 is usually associated with the people set apart for God, such as the 12 tribes and the 12 disciples. The number 40 indicates a long passage of time, such as the 40 days and nights of rain in the flood, the 40 years of wandering in the desert and the temptation of Christ coming after 40 days. The number 666 is used as the mark of the beast in Revelation.