The Washington Post reports that the "National Counterterrorism Center maintains a central repository of 325,000 names of international terrorism suspects or people who allegedly aid them, a number that has more than quadrupled since the fall of 2003, according to counterterrorism officials."
The NCTC refused to say how many of them were US citizens, saying only that it was a "small fraction" of the total number. It's not clear what the criteria used by the adminstration to consider someone a "terrorism suspect is".