The number 666 is only mentioned once in Revelation 13:18
This calls for wisdom: let him who has understanding reckon the number of the beast, for it is a human number, its number is six hundred and sixty-six.
whatever this means is still being debated on, and not final conculsion had arrived
Some says this is the mark of the beast, from verse 11-18, this seems to be the mark of the second beast (which seems to be the spokesperson of the first beast),
the only clue given is that its a human number (NASB) or the number of a man (KJV).
The saying of the number 13 has no barring/basis whatso ever, so most likely an old wives tale (one version of this is that there are 13 person in the upper room the night when Jesus is betrayed, and also Jesus was believe to be crucified on the day before sabath (ie. Saturday) making it a Friday.
Hence Friday the 13th saying)
Anyway, I personally do not think we should put too much weight on these numology stuff, its may be fun to know, but not sure if its benefical at all.