The number 666, is largely inspired by a passage from the Bible, specifically Revelation 13:18. The following is the passage:
"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 also known as The Number of the Beast, and the occurrence of the number in everyday life can summon feelings of foreboding to superstitious minds, similar to the number 13.
Sources cited: The Companion Bible; page 1901.