Personally, I like Michael Ward’s theory that the books were patterned after the seven planets recognized by medieval astrologers, and the characteristics of the deities associated with them: the sun, the moon, Mars, Mercury, Jupiter, Venus and Saturn.
FWIW, I list them in this order because that is the order after which the days of the week have been named, though the English names are a weird mix of quasi-English terms (“Monday” = “Moon’s day”), Latin deities (“Saturday” = “Saturn’s day”) and Norse deities (“Thursday” = “Thor’s day” = French “Jeudi” or “Jove’s day”). Also, the word “planets” comes from a Greek word meaning “wanderers”, and these were the only seven objects visible to the naked eye that “wandered” in the sky; the outer planets Uranus, Neptune and Pluto were discovered in just the last few hundred years.
And I think it’s interesting to see how this debate has spilled over into discussions of Harry Potter. IIRC, Richard Abanes has suggested that there are seven books in the Potter series because J.K. Rowling is influenced by astrology, which to him is bad, whereas John Granger has praised the books for precisely this sort of Christianized medievalism. I cannot recall if Granger has ever addressed the astrology charge, per se, but I imagine he would approach this the same way he approaches the books’ approach to alchemy, which is something else Abanes frowns upon.
FWIW, I would not be at all surprised if Rowling, an admitted fan of Lewis’s, had planned seven books to match Lewis’s series. Then again, since each of the books takes place over the course of one school year, perhaps there are simply seven years in the British schooling system, and we should just leave it at that!