If my opponent has cast 2 spells and i {Flusterstorm} his 3rd spell, at the end of the turn I cast {Brain Freeze} to mill his library; how many copies of {Brain Freeze} are there?
I am by no means positive here, but I believe you will get 4 copies + the spell itself for a mill of 15.
Your opponent cast 3 spells. Even if you successfully counter his 3 rd spell with your fluster storm, casting is the act of puting it on the stack and will count towards storm. The copies from storm are just that, copies. They are not cast, and so won't count towards future storms.