Statistically, around 0.005 per year (note that there's a HUGE range of estimates for how common defensive gun use is, so there's definitely some error bars there). However that's still many orders of magnitude more than the average number of mass shootings that the average civilian gun owner is involved with.
I think part of the intention of the Second Amendment is to have a well-armed population that has the capability to push back against a government that has overstepped. Five or ten rounds isn’t going to help much there.
