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?