From the Journal of Criminal Law and Criminology, we learn that every year, people in the United States use a gun to defend themselves against criminals an estimated 2,500,000 times, which breaks down to about 6,500 people a day, or once every 13 seconds. Of these instances, 15.6% of the people using a firearm defensively stated that they "almost certainly" saved their lives by doing so.
The FBI did a study titled “Critical Incidents in Policing” which tells us that out of 83.5% (2,087,500) of these successful gun defenses, the attacker either threatened or used force first.
Finally, a National Crime Victimization Survey done over a period of several years concluded that 76% of all uses of guns as a means of self-defense never involve firing a single shot.