Six is obviously the number of man, and I don't think we are being told the number six hundred and sixty six but rather 6 and 6 and 6 as in a 3 man trinity.
The first beast comes out of the sea, and sea is typical of peoples or gentile nations in the Bible, this man (Antichrist) is also represented by other types of men in the Bible.
Like Nebuchadnezzar, he was a ruler of the earth put in place by God, remember how it was God that gave him a vision of a image of a man, that represents Nebs and the other coming kingdoms.
And Neb built himself a statue of the image which was also in his likeness that was 60 by 6 cubits. And made a decree that all must worship the image of himself.
This story isn't in the Bible solely to just give us an interesting history lesson, which it does, but more importantly like all Bible stories it's foretelling future events.
Another world ruler, the Antichrist, a man who starts off being upright and good moral character is going to end up in the position of world ruler
What I mean by being of good character is that Nebuchadnezzar was of good character, a good representative of man, good enough that he put Daniel under himself, and had moments where he gave glory to God. ((I realize he did bad as well but that's another angle for later)
This will be similar to the Antichrist, he will be a good respected man of the world, but also when he does come to power it will be with 10 nations as allies and a peace treaty, which kicks off the seven years and begins the tribulation period
The second beast rises out of the earth, which coincides with the opening of the pit, and the king of the demons will be released and will enter into the first beast aka the Antichrist and will again Co-rule the world, the Antichrist will be mortally wounded at this stage, and a great miracle will have been seen to take place, now the Antichrist will be seen as the prophet of God.
Remembering our old testament story's again Moses the prophet of Israel when he died and his eyes were still bright,(not long dead) Satan tried to enter the body, but he was not allowed by God. Satan was not to rule Israel but the nations only.
Anyway for those who can stomach it, Satan was also a man, when he was ruler of the world anyway, remember The prophet Isaiah was sent to the king of Babylon to give a message to Satan, yes it's the one and the same king/man/Satan.
Thirdly the Antichrist builds himself a statue of himself, probably about 60 cubits tall, whatever that is in today's measurements, and will demand the world give homage to him or face death.
Not only that but this image will come to life and speak and talk, this big talking man seems too incredulous for me to believe that it's even a possible reality but then, if thats what God says is going to happen then I just will accept it even if I don't comprehend it yet.
Rev 13:5 And he is permitted [also] to impart the breath of life into the beast’s image, so that the statue of the beast could actually talk and cause to be put to death those who would not bow down
and worship the image of the beast.
Any way I'm purposely being brief on detail as this is such an intertwined topic, and just wanted to say enough to explain what I see the 6 6 6 is, which is simply three representatives of unbelieving mankind.
Three men also three beasts. The Antichrist who becomes accepted as a prophet of God/false prophet, Satan who is allowed to give power and life to the false prophet to do signs and also give life to the third beast the image that comes to life.
This explanation of 666 seems to line up with the other harder to understand explanations to me.
For example at the end of the tribulation, the false prophet and the beast are cast alive into the lake of fire, this the first beast the Antichrist, and the third beast the statue that comes to life, while the second beast(Satan) is put into the pit for a thousand years to be released again to gather the armies again against Israel again.
Hopefully this simple explanation makes sense

and hey if you don't agree hopefully you enjoyed reading someone else's far-flung explanation of things.