Hey Viet,
Glad to hear from you. Yes, every post in the Forum brings you a Euro discount :)
Anyway, the number is exactly from my experience. With specific acceptance criteria, I know how many other currency pairs I would need to test.
But this does not mean you have to use exactly 9. The number is not that important, but the pass of the Multi Market.
If you use 6 for example, and 4 of them pass that is good enough as well.
Here it is just about checking if the strategy is over-optimized for the current asset, or it works on others as well.