The Volokh Conspiracy
Mostly law professors | Sometimes contrarian | Often libertarian | Always independent
You can see the list from the Crime Prevention Research Center; it goes back more than a year, but I counted the incidents just since June 4, 2021. The list is supported by links to media coverage of each incident, so you needn't trust and can instead verify. I checked a few and they seem to check out, though of course there's always the possibility of error in news coverage (plus of course there's no reason to think that the list is complete, since there may well have been such incidents that weren't covered in the news in a way that would come up on the Center's searches).
Naturally, it's also hard to tell how the incident would have played out had the defender not interceded; it doesn't cover the almost certainly far more common scenarios of self-defense against non-mass shootings (and of the non-mass shootings themselves); and of course this doesn't tell us whether some particular gun control proposal might, on balance, reduce underlying gun crime in a way that doesn't unduly interfere with lawful self-defense. Still, I think it's worth noting these incidents, to help us keep in mind the possible costs of gun control measures that do unduly interfere with lawful self-defense.
Here's one incident from the list, from Syracuse (N.Y.), WSYR-TV (Natalie Dascoulias):
Demetrius Jackson, the man killed in the Lodi Street shooting on Tuesday, was in possession of a loaded 9mm handgun while outside of 1808 Lodi Street, District Attorney William Fitzpatrick said.
Jackson threatened multiple people at the location and fired the loaded handgun in the direction of those people, officials said.
Another man, who was on scene and in possession of a 9mm handgun returned fire striking and killing Jackson, Fitzpatrick said.
According to Fitzpatrick, this man has a valid pistol permit for the 9mm handgun.
The District Attorney said in a statement that based on preliminary investigation, it appears the man who shot Jackson saved the lives of several individuals.