Alice A. Bailey had some interesting things to say about the number 666, which most know from the book of Revelation in the Christian New Testament.
“Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.” -Revelation 13:18 (KJV).
Many mainstream Christian theologians and historians see this as a reference the Emperor Nero famous for his persecution of Christians. “Nero Caesar” through various forms of gematria can give the number 666 (Wikipedia).
Reading Alice A. Bailey, quotes below, her take is more on astrology, rays and energies. For example in “Rays and the Initiations” she writes “… 6 being the number of the sixth ray, it is therefore the number of idealism and of that driving force which makes mankind move forward upon the path and in response to the vision and press upward towards the light.”
Within this sum total of active, qualified energies, we have (working out today) influences and impulses from three major constellations. These pour through certain other related constellations and planets into the three major planetary centres: Shamballa, the Hierarchy and Humanity. It is with these three centres I would seek to deal and to them and their inter-relations continually make reference. I would ask you to bear in mind that these energies are transmitted from one point to another, or pass through with transmuting effect from one centre to another, thus bearing their own vibratory quality but carrying also that of the centre of transmission. In the transit of energy from a major centre to a human being and when the energy is finally grounded in the disciple’s centre, you will notice that it is then a fusion of six energies. These three groups of energies (each of them a fusion of six) make the dominating energies, controlling the human being, to number eighteen; this holds the clue to the mystery of the “mark of the beast” which is 666. This is the number of the active intelligent man and distinguishes his form nature from his spiritual nature which is 999. These three lines or streams of energy in man may be listed as follows: