The answer is none, and the math says the chances of finding one are well over one million to one. It's not like Jihadi's take selfies with a bomb strapped to them to show their friends on facebook.
I have no idea, but I think it's safe to say at least one attack would have been stopped, and if that's saved lives.... I dunno, at what price is freedom?!
Dunno, how many have you decided?
How many terrorist attacks do you think have been stopped because of this surveillance?