I was turning 34 with my last cycle, and they retrieved around 19, and 13 fertilized. With my cycle a few years ago, 27 were retrieved, and 22 fertilized. I think a lot of it IS just chance, cycle to cycle, but in my case, the protocol was exactly the same, so I really believe age had a lot to do with it.
Don't blame yourself or question what you did or didn't do - I don't believe, for the most part, there's ANYTHING we can do. We have to trust that our doctor's recommended protocol is the right one (or question them if we have doubts). After that, it's all up to the meds and our bodies. We can only control so much. As I said before - stay hopeful!! 9 really is a good number to work with!