It seems that it is the clouds themselves, rather than the number of them, that were in the thoughts of those who coined this phrase. The imagery was originally of a 'cloud cuckoo land' or 'head in the clouds' dreaminess, induced by either intoxication or inspiration, rather than the 'idyllic happiness' that we now associate with the phrase. (source) As mentioned above the figurative use of it now is that of a blissful state of happiness - sometimes so much so that we miss reality happening around us! I confess - I look at the bliss - the cloud nine feeling these pictures convey - and I feel jealous. Have I ever felt that free? Have you?

This phrase was authored by John Milton in his 1634 work  Comus: A Mask Presented at Ludlow Castle.   I see ye visibly, and now believe That he, the Supreme Good, to whom all things ill Are but as slavish officers of vengeance, Would send a glistering guardian, if need were  To keep my life and honour unassailed.  Was I deceived, or did a sable cloud  Turn forth her silver lining on the night?  I did not err; there does a sable cloud  Turn forth her silver lining on the night,  And casts a gleam over this tufted grove. The phrase morphed throughout the years to the one we use today. Read about its evolution here .  One afternoon I was driving and spotted the silver lining.  So I had to stop and capture it!