>Who is claiming any measure will reduce risk to zero?

No one. But some claim that the current risk must decrease, while it is much smaller than almost everything else.

The current risk is measured while the border patrol and other agencies do their job.

How many terrorist attacks would there be if no one was trying to stop them?

Perhaps there would be (almost) the same number of attacks. If you disagree you have to provide evidence in order to break into privacy of others.

BTW, NSA never prevented any terrorist attack: https://en.wikipedia.org/wiki/Global_surveillance#Purposes

Upd: Also, if the risk is already small, why we need even more privacy invasions?

Unfortunately the tools available to terrorists continue to change and so must the measures used to fight them.

> NSA never prevented any terrorist attack:

> The director of the National Security Administration today told Congress that more than 50 potential terrorist attacks have been thwarted by two controversial programs tracking more than a billion phone calls and vast swaths of Internet data each day.

http://abcnews.go.com/Politics/nsa-director-50-potential-ter...

Thank you for the link. But I wish I could believe that after so many unpunished lies from them...

https://www.theguardian.com/commentisfree/2013/sep/25/nsa-re...

> Unfortunately the tools available to terrorists continue to change and so must the measures used to fight them.

From what I know, the current problem is not the lack of information, but how well it is targeted. For example, we knew about the terrorists in France in 2016: https://en.wikipedia.org/wiki/March_2017_%C3%8Ele-de-France_...

Also, many think that the war on terror made more harm to the humanity then what we would have otherwise. At least such invasive war.

https://en.wikipedia.org/wiki/Criticism_of_the_War_on_Terror

Not exactly a disinterested party, is he?