One classic explanation from bible scholars is that it may be intended in contrast to the number 7, a “perfect” number in biblical symbolism. Three sevens, 777, would signify the holy God known to Christianity that comprises Father, Son, and Holy Spirit. So 666 would mean the attempt of the sinful world in collusion with Satan to attain to godhood, as in the sin of the Garden. The book of Revelation is deeply couched in mystical terminology, as if that needed to be said. So the literal usage of 666 like that might never take place, any more than the churches of Asia had literal lamp stands.