Givens small report seems to suggest that capacity is rarely important in gun fights. All the statistics, from every source, seems to always fall around 3-5 shots fired in almost all encounters.
As much as some claim, and even state outright, "capacity is the most important factor", all evidence and statistics always disagree.
In my experience most folks don't understand data, what it really means or what conclusions can logically be drawn from it - and what things cannot. Thus, they don't trust data or statistics, but rather rely on people they see (sometimes legitimately, often not) as authority figures.
People also suffer from both "anchor bias" and "confirmation bias". People tend to believe the first thing they hear from someone who they regard (rightly or otherwise) as an "authority". Unfortunately that information is sometimes taken and or presented out of context, and or lacks necessary qualifiers, and or is based on erroneous conclusions from data.
When that happens it makes absolutely no difference. People continue to believe it because it's the first strong opinion they formed. Thus the "anchor" bias.
They build on that false assumption and related opinion(s) by seeking out information that agrees or confirms their opinion, while rejecting information that doesn't support their opinion. That's the "confirmation" bias.
For example, someone might read about an officer involved shooting where more than 15 shots were fired and a spare magazine was needed. Despite that being a rare occurrence - around 4% of all officer involved shootings, even when officers go looking for bad guys in bad neighborhoods. An armed citizen won't see the differences or the low rate of occurrence and might conclude they should carry a high capacity semi auto and a spare magazine "just in case", without considering the downsides.
Then they read an article like the Givens article where again the data clearly supports the concept that 90+ percent of defensive shoots will be adequately addressed with 3 or 4 rounds, and similarly noted that only 3 of 63 shoots (about 4%) required significantly more rounds (8, 11 and 12), with no one needing to reload. However, they'll see those 3rd standard deviation tail of the distribution statistical anomalies (with no explanation of why more shots were fired, and or whether they were even necessary, and or the result of poor shooting) and conclude they should err on the side of caution and carry a high capacity semi auto and make that a priority in their defensive handgun selection.
Interestingly people stuck in confirmation bias mode will almost certainly ignore the data showing 3 out of 66 students "forfeited" by being unarmed. At a minimum that should raise the question of whether those three may have been armed had they chosen a lighter, smaller, more concealable firearm. Again that's about 4% and again statistically in the tail of the distribution. None the less, ot should be just as compelling as the "need more than 3-4 rounds" data. But it's not viewed that way as it doesn't support their preferred opinion, so it's just rejected.
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The thing is you won't move people off those established opinions with facts, data, or rational arguments, and when you fail, people generally just dig in deeper with their beliefs.
The best you can do is to engage them in a conversation and listen for a switch from "sustaining" their belief for things that suggest doubt or uncertainty about their belief and then have them expand on that "change" talk. You are essentially getting them to identify their own motivation to change.
For example I'm a die hard 1911 guy and it was a major change for me to accept that I actually liked CZ-75s. They fit my hand, I like the DA operation for a number of reasons, 9mm hollow points are more effective than they used to be, etc. Openness to those positives about the CZ-75 eventually led to a motivation to try one, then buy one, then carry one.
I am however not a Glock / striker fired tactical Tupperware fan. Still, if you get me talking about Glocks, you might get me to say "the odds of me carrying a Glock are pretty small". The approach then is to ask me why I didn't say the odds are zero, which will draw me out in terms of talking about their good points - and they do have a few. It won't be an immediate win, but at least you've got me open to a remote possibility and over time as I see things that support the possibility of changing my opinion, the odds of me changing my opinion become less remote.
You can do the same thing with someone who insists on a high capacity semi-auto and two spare magazines for EDC. When you get him talking about it, you'll eventually get him talking about the things he doesn't like about it, such as "the whole package weighs over 4 pounds and by the ne of the day my back hurts", or "it's totally impractical when we go to the beach", etc. That's at least an opening to talk about what factors might get him to consider backing off to a more comfortable EDC and or smaller, lower capacity handgun.
Once you get people open to new ideas, it helps moves them off their anchor and confirmation biases and they do open up a bit to new ideas and different ways of looking at issues.
The rest of it is just letting the new ideas percolate to the point they don't automatically reject ideas that differ from their previous opinions - and leaving them an out where they can evolve to a new position without having to admit they were "wrong".
Sometimes it's a matter of getting them to separate conflated beliefs. For example many 2A folks will reject the data saying you don't need 15 rounds in a self defense handgun because that could be construed as supporting the anti-gun argument that "no one needs more than 10 rounds" in any handgun or rifle.
One doesn't really have anything to do with the other. I have a right to own, shoot and carry an MP5 with a half dozen 30 round magazines, but that doesn't make it the ideal EDC for me.
