I would imagine that 10 bags would be more than enough. That's just my humble opinion though. I usually rotate between three bags (one for work, one for nights out, and another to run errands around town). I tend to find something that I like and then I stick with it. Right now I'm so comfortable with my speedy 30 that it's become my default bag. I use it as much as possible.
My best friend tells me that this will change once I begin to make more money! Who knows....