How many gun owners are there in the US?

According to the Bureau of Alcohol, Tobacco, Firearms and Explosives, there were about 215 million privately owned guns in 1999. The NRA estimates that half of all American households (total US population is 294 million) have at least one gun owner.
Numbers vary, but estimates are 70-80 million gun owners with 200-250 million guns.
There is no way to give a definitive answer. There are millions of guns in the U.S. Some estimates say 80 million households have guns, but since there is no national registration, there is no real way to know.