The 666 is the number of a man called the beast who will get cast into the lake of fire when Jesus returns.
I agree that, in reference to the Sea Beast, Revelation 13:18 calls 666 the "number of a Man". But pay careful attention to how the Sea Beast's conglomerate history of empires begins.
We know that this Sea Beast was said to have leopard, bear, and lion features (Revelation 13:2). That means the Sea Beast had a history as old as the lion empire of Daniel - which was the Babylonian empire under Nebuchadnezzar.
Look at how Daniel 7: 4 describes that first "lion" empire of the four beasts coming out of the sea. "The first was like a lion, and had eagle's wings: I beheld till the wings thereof were plucked, and it was lifted up from the earth, and made stand upon the feet AS A MAN, and "a MAN'S HEART was given to it".
Those whom John was addressing who knew their Daniel scriptures would have remembered this about the lion empire becoming like a MAN standing upon his feet, with a MAN'S HEART given to it. Nebuchadnezzar was the beginning of this conglomerate Revelation 13 Sea Beast. It eventually had a combined, calculated number of 666 years that the Sea Beast had exercised pagan empire control over Israel's affairs, up until the year when John was writing Revelation in AD 59/60.
In John's days, the Sea Beast had developed into its final phase (with Roman power this time over the nation of Israel), and was at that point in its 666th year of existence. Satan as the "Prince of this world" had worked behind the scenes of all those former pagan empires of Chaldean, Medo-Persian, and Greek, continually trying to disrupt God's plans.
In the final Roman phase of that Sea Beast's power, Satan had given his own throne in the city of Pergamos (Revelation 2:13) into Rome's control. The Roman Senate records tell us that the entire kingdom of Pergamum was given to the Roman Republic in 133 BC by the dying King Attalus III, who had no living heir to whom he could pass his kingdom. So "Satan's throne" in the city of Pergamos (the 40' tall altar of Pergamos in the temple to Zeus where Antipas was martyred) along with the whole kingdom and capital at Pergamos was bequeathed by Attalus III to his ally, the Roman Republic, in hopes that this would ensure a peaceful transition of power.
All the kingdoms which had composed the Revelation 13 Sea Beast were not simultaneously cast into the Lake of Fire - that was the other
Scarlet Beast of Revelation 17 along with the
Land Beast of Revelation 13 which were both thrown alive into Jerusalem's Lake of Fire for destruction. It was Satan and the entire demonic realm (which had operated behind all the ancient Sea Beast's pagan kingdoms) which was all crushed simultaneously into dust together, blown away on the winds after one strike of the Stone kingdom. Since then, Christ's Stone kingdom has been continuing to grow through the centuries, and will eventually "fill the whole earth" with its effects.
If you are looking to put a single name of a man on the Antichrist (the Man of Lawlessness), this can be done, but this single man was not the equivalent of either the Sea Beast, the Land Beast, or the Scarlet Beast themselves. All were different characters. The single man who filled the Antichrist role has been dead since AD 66, by the way - no threat to our generation at all.