This is always a hard one. I've usually tried to be (fairly) honest when a new girlfriend or girlfriend-to-be has asked me this. But before I answer I always counter with a question or two - Why do you want to know? and Will you tell me how many men you have been with?
As you say, not all girls ask. I don't ask them, either, unless they ask me. (Good advice from the court lawyer who said, 'Never ask a question unless you have a good idea what the answer's going to be AND you know that you want to hear that answer'.)
If I were you, I definitely wouldn't say 9. Depending on your age, it sounds low and it also does sound as if you are deliberately trying to keep it to single figures (on the same sort of basis that goods going for $9.95 or $9.99 somehow sound less expensive than they would if the price was $10.00).
Whether you think that 20 or 30 or some such number will sound 'better' than 45, well that's up to you. I suspect that it won't. I think the best thing is to be truthful - after all, a lie is bad basis to start a relationship on. I agree that far more important than 'numbers' is whether you've been faithful while you've been in a relationship that is supposed to be monogamous. You say you have, and I think that's what you should be emphasizing to her. Anyway, from what you say it doesn't sound as if she lets the grass grow under her feet, so you have a good chance that she won't be too bothered. If she is bothered, well maybe she's not the right person for you.
Whatever you decide to do, best of luck!