If your Mastery is 2, you get one original and two copies. They all have to be the same spell. And yes, you should need to defend against it three times (or create a defense that lasts longer than momentary). Overkill is a common design theme for magi of House Flambeau. But there is always someone bigger and badder out there.
Umm, I think with a mastery of 2 you get 2 spells... It says that: "You can cast several copies of a single mastered spell so that it affects more people, objects, or areas (as applicable) than normal. .... You may cast a number of copies of the spell equal to or less than your mastery score." I can see your reading of it, but I don't think that's right. I don't think you are getting the copies and the original. I think the number of copies is how many you have total.
No, this is incorrect. Marko is correct, though he didn't state it as a limit to how many you get - you can get fewer copies if you'd like. You have the original + mastery number of copies as a maximum. It was clarified in the errata: