6 is not the number of the devil. 6 is the atomic number of carbon. Meaning it has 6 protons, 6 neutrons, and 6 electrons. 666 - we are carbon life forms.
Who am I to argue with The Pixies:
If Man is five,
If Man is five
If Man is five,
then the Devil is six
Then the Devil is six,
the Devil is six
The Devil is six,
and if the Devil is six
Then God is seven,
then God is seven
Then God is seven
The Pixies - Monkey Gone To Heaven