I did a search on it. You get different numbers. Some sources say 45% to 50%.

No, the closest thing to an ID card in the US is either a drivers license (which does not prove citizenship, any legal resident can get one) or a Passport (which does require citizenship by review of birth certificate).

Any US citizen who does not have a Passport (or Certificate of Citizenship/Naturalization) has no document proving their citizenship. A very high number of US citizens have no documentation to show their status as citizens.

No papers? Enjoy your stay at Camp South Sudan.