Stan, Though once considered the most logical choice for the NSSA game #9s are less favored by top shooters of late. Last I read Bender's thoughts he was using #8s for 20/12/doubles. I can't remember if he favored #8.5s or #8s for 28, but it wasn't #9s. I think #9s are still favored for the 1/2 oz payload. He's not the only top shooter to have moved away from #9s. Some have held fast.
This is not to say that #9s are a poor choice for mere mortals who do not center literally every target. If you can put every skeet target within 3 inches of pattern center you can get away with #6s, I imagine. The 21 yard standard simply relates to center stake. For other than top tier shooters, reliably getting that 2nd 3,4,5 doubles target before it reaches the center stake is a trick.
I disagree with the proposition that #9s give reliable single-pellet breaks.
Sam