Actually, the correct answer may be: None.
The actual Greek John used may mean more than just six-hundred and sixty-six.
Each of the three Greek symbols is different than the other two. Yet, each is translated as a 6!
In fact, one of the symbols looks nearly exactly like the Arabic script for "Allah", rotated 90 degrees. Go to this site, and you can see the original document:
Note: Click on #300 GA_03_0150b.jpg
Please visit the website link at the end of this message to see the ancient greek called Codex Vaticanus. When you click on the link, scroll down and click on #300 GA_03_0150b.jpg. Once the picture opens you will find a snap shot of the Revelation chapter 13 and part of chapter 14. Please click on the picture to zoom into the text. It will be easy to identify each verse as you zoom in because you will notice that each verse has a hand written number. You will find Revelation 13:18 to the far right of the picture. At the end of Revelation 13:18 you will see this "????".