Obviously, "nine" denotes a constant; the number "nine" doesn't change. (Even when the number of planets changed from nine to eight, the number nine itself didn't change at all.) Which is why I was particularly pleased to see the following quote in the New York Times, from Dr. Elizabeth Blackburn, one of this year's winners of the Nobel Prize in Medicine:
Only eight women had won the Nobel Prize in Physiology or Medicine. Asked how she felt about becoming No. 9, Dr. Blackburn replied, "Very excited, and hoping that nine will quickly become a larger number."