According to the Oxford English Dictionary (Second Edition), there are 600,000 words in the English language.
Each of the six slots can contain any of those 600,000 words. So the total is 600,000 to the sixth power.
Of course this total number does not include numbers, special characters, foreign words, and totally made-up words, which if included would drive the count much higher.
But wait, you say. "Many of those Sixes would not make any sense!"
That's ok, neither do some of Picaboo's Sixes. :-)