This is a very difficult thing to measure, because you have to find events that were not mass shootings, but could have been.
Someone is probably trying to keep track of it somewhere, but you won’t find it on the FBI Uniform Crime Report.
The same is true for many, if not most, instances of self defense with or without weapons. Averted outcomes are not something that get measured well. We’ve tried, and even the most conservative estimates put defensive firearm use at least on par if not far, far exceeding the instances of criminal firearms uses.
This was confirmed as recently as an Obama era CDC study.