The number '7' is a number often used to lend a sense of validity and sacred mystery to poems, monographs, bibles, and the like.
In the The Secret Doctrine, creation passes through seven 'rounds' or evolutionary cycles, four of "descent" and three of "redemption" (Goodrick-Clarke, 1992).
In Zoroastrian face, "the first of Ahura Mazdā’s creative acts was to emanate the six great Beings known from the tradition as the Aməša Spəntas, who likewise are aspects of his own being, and with Spənta Mainyu make up a mighty heptad." (Boyce,2011).
Boyce, Mary. (2011). Ahura Mazda. In Encyclopadia Iranica. New York: Columbia University. Original edition, 1984.
Goodrick-Clarke, N. (1992). The Occult Roots of Nazism. New York: New York University Press.