There are allllll kinds of things about the number 9. You can check them out on Wikipedia and other sites. http://en.wikipedia.org/wiki/9_(number)
Here's a little music trivia that has to do with 9.
First you have to remember that 3 is the number of the Trinity. Therefore it was considered a good meter (or time signature) for sacred music.
If you take the beat of a song in three time and divide the beat into two you get 3/4 time. Examples would be "Come, Thou fount of Every Blessing" and "Ancient Words."
If you take the beat of a song in three and divide the beat into 3 you get 9/8 time. This of course then would be very sacred back in the days when numbers meant so much and I've heard is the reason Bach used that double Trinity time of 9/8 for his beautiful song "Jesu, Joy of Man's Desiring,"
Here's an interesting fact you may not know - possibly because I'm making it up even as I type. When there are nines in the date on a Wednesday it means it's the perfect time to be in choir! So with today having 3 nines in the date is is like triple wonderful time to be in choir. How cool is that? I'll be looking for you tonight!