9 sets is just fine. Recent research seems to suggest that 10 is optimal (some gains after that but returns diminish quickly).
Take a look at the work by Dr. Pak (Androulakis-Korakakis) on minimum effective dose training. I might get maybe 3-6 sets of chest a week deepening on what wrist is going on and I'm still making gains.
I like to go with as much volume as is necessary to get a positive response. If my reps are going up and/or RPE is going down I know that I'm making progress and that whatever volume I'm using is adequate. That might not be optimal, but I don't get burned out this way, and if things start to stall I have room to increase volume further if I need to. That's just my preference, though.