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.”