300 Million
At the current clip of one birth every eight seconds, a death every twelve, and an immigrant entering the country every thirty-one, the 300-millionth American should show up, one way or another, sometime [in October]. The 100- and 200- million marks were eclipsed in 1915 and 1967 respectively.
I'm glad to report that my wife and I have done our part to help the U.S. reach this level.