Revelation 13:18 is the verse.
Most Biblical scholars think that the likely candidate for who this infamous number represents is the Roman emperor Caesar Nero whose name in Hebrew numeric value gives the required sum. Nero personifies the emporers who viciously persecuted the early Christian church.
Scholars who believe the Book of Revelation refers to real people and events argue that the number represents the value of Nero or Neron Caesar (ðøåï ÷ñø, Nrwn Qsr*, pronounced "Nervon Qwaesar") in Hebrew letters, which also have numerical values (note that Hebrew is written from right to left):
r S q n w r n
666 = 200 + 60 + 100 + 50 + 6 + 200 + 50
As noted above, some early texts of the Book of Revelation use 616 instead, which would represent the alternative Hebrew spelling ðøå ÷ñø, Nrw Qsr (based on the Latin form "Nero Caesar"):
r S q w r n
616 = 200 + 60 + 100 + 6 + 200 + 50
It is unknown why q (qoph) is substituted for k/c (kaph), or why one vowel is represented where the others are not and represented by the consonant w (waw).
https://en.wikipedia.org/wiki/Mark_of_the_Beast
Also, the fear of the number 666 is known as "Hexakosioihexekontahexaphobia".
