Revelation 13:18 (King James Version)
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.
Some old versions of Rev 13:18 do not show 666 but 616 and 665.
KJV spells out the number but in the Greek it is actually shown as Greek numbers.
I understand Irenaeus knew about these other numbers and rejected them in favour of it being 666.
This is discussed in these links
666 wrong number of prophetic beast?
Number of the Beast - Wikipedia, the free encyclopedia and Wiki has a lot more links
The Other Number of the Beast: numerology
The interpretation using numbers corresponding to letters has always come up with strange results. I remember around 1989 it was Mikhail Sergeyevich Gorbachev depending on language and how you used his name such as in Russian it had to be Mikhail S Gorbachev.
In the other articles they talk about Nero and having to call him Neron and that it could have meant Caligula etc etc
And not many people know this verse in the Old Testament
1 Kings 10:14-22 "Now the weight of gold that came to Solomon in one year was 666 talents of gold".