Why is six Satan's number?

Answer 1

The numbers 666 represent three biblical events. Satan is the third event [6]. This is when he will be let loosed from Heaven and begin his reign of terror on the earth. The book of Revelation illustrates this event as a pouring out of the 6th vial.

Answer 2

John, in Revelation, told a first century audience that with wisdom and insight they could calculate the number of the beast, for it is a man's number. Transforming names into numbers is called gematria and was a common practice in John's time. The readers of John's letter familiar with the Hebrew language could recognize that the Greek spelling of "Nero Caesar" transliterated into Hebrew equals 666.