Framework for understanding what's going on here
Essentially this is political theater.
The senators asking these questions at an open hearing know very well the classified details of what the NSA is actually doing under established law and interpretation, which the Obama administration supports, carried over from the Bush administration which initiated all this after 9/11. (Not that I personally know any of these classified details, just to be clear.).
They just don't agree with those laws and interpretations. But they know they can't win repeal outright (far from it, actually). So this campaign is an attempt to reframe the discussion and to generate media attention by trying to drag classified details into the open in a back door way.
"But it's such a simple request! Just tell this committee, in an open hearing, details about how many Americans were involved in intercepted communications?"
Really? That's not going to happen. And, THEY KNOW THAT.
So they force an answer like the one they got. Then they can fire up the media just like you see happening.
Don't get spun by this. Understand the true nature of this campaign. Whether you agree with these Senators goals, or not. Debate the issues on their merits, not some ginned up "controversy".