According to Wikipedia:

"... The Number of the Beast is described in the Book of Revelation 13:18. From the King James translation: Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six ...

"... 666 is an abundant number. It is the sum of the first 36 natural numbers (i.e. 1 + 2 + 3 + ... + 34 + 35 + 36 = 666), and thus a triangular number.

Since 36 is both square and triangular, 666 is the sixth number of the form n²(n² + 1) / 2  ...and the eighth number of the form n(n + 1)(n² + n + 2) / 8 ...

... 666 is the sum of the squares of the first seven prime numbers (i.e. 2² + 3² + 5² + 7² + 11² + 13² + 17² = 666).

... The Roman numeral representation of the number 666 (DCLXVI) uses once each the Roman numeral symbols with values under 1,000, occurring in descending order of their respective values (D = 500, C = 100, L = 50, X = 10, V = 5, I = 1) ..."

We can add that 666 = 354 * 2 - 42, where 354 = 12 * 29.5 (the length of the lunar month). Beyond a lunar year (354 days) the order created by Man (tagata) reaches no further - it is a time for the Beast.