Here is WISDOM.
This is WISDOM for the reader, and its source is clear (Proverbs 2:1-6). It is not from the beast or the devil.
This expression is used after Daniel's writings (Daniel 12:10) and again by John in discussing the beast's seven heads and the great harlot (Revelation 17:9). Ironically, the WISDOM in this verse has caused more discussion and speculation than perhaps any other verse in the Revelation. Sad to say, the world (and probably most believers) know very little about the Revelation but almost all have heard about 666.
This is not a parable but more like a puzzle. It will be revealed when necessary just like Jesus explained why he spoke in parables (Matthew 13:11).
Let him who has understanding calculate the NUMBER of the beast, for the NUMBER is that of a man; and his NUMBER is six hundred and sixty-six.
This discussion will not go into the many attempts to 'calculate' the NUMBER, or do a gematria to determine whether a leader (past or alive) equates to 666. It is possible that the 666 may not be a calculation but a physical symbol like 3 coiled up spirals used in security tags in stores and libraries. The calculation becomes the unique resonant frequency of the security tag.
The number six, itself, is interesting in that man was created on the 6th day (Genesis 1:24-31); the archenemy of Israel, Goliath, was 6 cubits and a span, and had a spear head weighing 600 shekels, and wore 6 pieces of armour (1 Samuel 17:4); and lastly, the foulness of the devil is demonstrated in the 6th bowl (Revelation 16:12-16). Perhaps this is what Daniel saw in Daniel 11:21-28 as directly impacting the world leaders.
Finally, if the 666 were some sort of ID chip to be inserted under the skin, Old Testament law forbids the Jewish people from cutting the skin 'for the sake of evil' or to have markings (tattoos). Yet those whose names are not written in the Book (verse 8), will accept the 666 NUMBER.