According to a recent report from the Congressional Research Service, data provided by the ATF says that there was at least 310 million guns in circulation in the U.S. as of 2009, including 114 million handguns, 110 million rifles, and 86 million shotguns. Private ownership is currently estimated at 270 million.
According to the same report, data provided by the Bureau of Justice Statistics suggests that possibly as few as 1 percent of victims of violent crime actually use a gun to defend themselves.
Official estimates for total number of guns in circulation as of today aren't available, but the best guess I've seen is 327 million. This is based on adding the total number of guns manufactured since the ATF published its 2009 report.
Needless to say, there are way too many guns in this country.