Also important to note a few things about this data, the frequency which people carry and the likelihood of the shooting happening in an area where one isn't legally allowed to carry.
According to this https://checkyourfact.com/2018/03/05/fact-check-what-percentage-of-americans-have-concealed-carry-permits/
Just 6.6% of Americans have a CCW permit. Some do also open carry, but the number can't be that much higher, and not all of those people even carry regularly, some only do sometimes, let's call it a generous 10-12% carry regularly. Even at 10%, that isn't very many, you're more likely to not have anyone armed around you.
Especially considering that most often, the type of mass shootings we're talking about are public mass shootings, not mass shootings at someone's house party that are gang related. Clubs, bars, schools, theaters, concerts, etc, are by and large areas where you're not allowed to carry. Even some stores like walmart prohibit carrying guns inside (and have had shootings before.) This is also going to lessen the likelihood that someone will be armed to respond. Depending on sources the numbers of how many mass shootings take place in said gun free zones varies wildly. If we're cutting out robberies and gang activity, John Lott at the Crime Prevention Research Center puts the number at 98%, if we're including the gangs, drugs, and robberies, Everytown puts the number at 10%.
For an armed civilian to respond, one of those 6.6% of people has to be legally allowed to carry, and have happened to bring their gun today, and even then they still have a gunfight to win they can easily lose. 22/433 is 5.08% of times an armed civilian was the one who stopped the crime, at 6.6% or even 10% of people carrying, I'm gonna say 5.08% is not that bad and the number could go up if more sane people would carry and be ready to save themselves and others should the need arise.