This is another of those questions with insufficient data for a precise answer. The major gun control drives of the last half century have so many people leery of pollsters asking nosy questions that it is doubtful that the actual number of crimes prevented each year can be determined by polling.
We do have the Wright Rossi’s Reports prison interviews, which concluded that “At least two million crimes are aborted each year because the victim may be armed.” But that was almost forty years ago, and it was limited to a few prisons in largely intensively gun controlled venues. Where and when there was far less reason for predatory criminals to give a second thought to effective resistance.
The hit and miss studies that have been undertaken since then come up with numbers as high as ten million serious crimes a year aborted because the victim may be armed. Most of those potential crimes are never reported to police; largely because the potential victim was unaware of the fact they were a target. But even there, the studies were essentially polling prisoners, who may have powerful reasons not to be fully forthcoming.
But sometimes a victim is forced to shoot at a predatory criminal. The estimated numbers are all over the lot, but they suggest that Americans actually pull the trigger on some 60,000 predatory criminals every year. Most of those shots seem to be warning shots, since only about 10,000 criminals are wounded each year. And between 15 and 20 percent of the wounded are killed. It is notable that predatory criminals who survive an “encounter” with an armed citizen have a very low recidivism rate. Those who do not survive have an even lower rate.
The “Equalizer” also works remarkably well for women. Rape is one of our most common crimes, but a number of studies agree that armed women and girls stop 450 rapes a day, 165,000 rapes a year, usually without firing a shot. (NB, these studies are all several years old. Large numbers of women have started to carry and it would not surprise me at all if the 2010 numbers were twice that.)
The bottom line? While there are still many things that are unclear, it is clear that American gun owners stop a huge number of crimes cold. Perhaps as many as twelve million serious crimes a year. Whatever the true number is, the United States would be a much more violent society if we were disarmed. As England adequately demonstrates.