Maybe what is needed is a discussion on what the role of a permit holder should reasonably be. Is it to stop the threat or is it somewhat more limited and to be a calm voice in a localized area; a guide and rear guard within a small perimeter limited to a 30 foot radius or so? Should we train to locate exit and shelter points in our immediate area rather than the charge to the sound of gunfire response?
We have had several such threads, at least in part. People get permits to carry for a variety of reasons, not all of which include self defense all the time. That a person (even without a permit) may be able to engage in legal lethal self defense of him/herself or of others (such as going after the a mass shooter) does not mean that they will or are obligated to do so. That is much more of a personal and situational decision to engage than a role that one is to perform.
However, what JohnKSa seems to be driving at is the notion that the gun community for years has cried out how if there was just one citizen 'there' with a gun, this mass shooting or that mass shooting could have been stopped. It is true, but not only do they have to be there with the gun to use the gun, but they have to actually engage the shooter. Often, the gun community wants to blame gun free zones, but we have had a bunch of non-GFZ mass shootings where there wasn't anybody there that fought back with guns, but a few where they have, often those people being LEOs (on or off duty), former LEOs, and in at least a couple of cases, firearms instructors (Tyler and in the last responding Sutherland Spring church shooting). Interestingly, in both the Tyler and Sutherland Springs incidents the responding citizens weren't actually part of the shooting, but injected themselves into the situations. The same happened at the Louie's Bar and Grill shooting in OKC where two responders (Nazario & Whittle) who were not in the shooting managed to each retrieve their guns from the trunks of their vehicles and both go after the shooter (Tilgham) and kill him after Tilgham took flight.
The gun community is quick to herald when somebody with a gun was present at such shootings. There were excited posts about Joe Zamudio who responded to the Giffords shooting with his pistol, intent to stop Loughner, only Zamudio showed up late and almost shot the guy securing the gun from Loughner AFTER Loughner had been taken down. We were excited to hear that Glen Oakley was at the Cielo Vista Mall when the shooting was in El Paso and how he was running to the sound of gunfire, but stopped to save the poor children who weren't with their parents. Mind you, the shooting wasn't at the Cielo Vista Mall at all and Glen Oakley was at the far end of the mall from the Walmart and had the mall between him and the Walmart that was some 500-600 yards away, but for a while, we were excited that we had a guy there who made a difference even though he made no actual difference at all that was remotely relevant to having a gun. We like to try to take credit for "off duty" officers as being normal, every day citizens who respond, which is interesting given that the off duty officers still retain all their police training that most average citizens don't have, and enjoy many situational and legal protections and capabilities that everyday citizens won't enjoy in those situations. We have heralded where the shooters were "stopped" by citizens with guns, even when all evidence points to the shootings actually being over (Pearl, MS, Sutherland Springs). Heck, we were excited when Dan McCown was at Tacoma Mall. Like Joe Zamudio, he made sure to get his 15 minutes of fame, talking about how he carried a gun for the defense of others, but when it came to brass tacks, McCown was afraid to draw his gun out of fear of getting shot by the cops (who weren't even there) and the fear of shooting bystanders, but took credit for stopping the shooter because after McCown stood up and yelled at the shooter and the guy shot him multiple times (crippling McCown for life), the guy didn't shoot anybody else. Similarly, we are excited by Nick Meli's claims of stopping a mall shooter. Meli, an off duty armed guard, said he drew down on the shooter who had stopped shooting because his gun jammed, saying the shooter retreated at the sight of his gun. Meli didn't fire because of the danger to others and the shooter later managed to clear the malfunction and commit suicide, but many in the gun community feel that Meli "stopped" the shooter.
Despite the increased number of permits issued across the country and the fact that 16 states have Constitutional carry where virtually all adults of age could carry guns, we don't see a striking increase in the intervention into mass shootings by all these people who could be armed. There are some, of course, but it isn't like the presence of all these opportunistic defenders are putting the fear of God into the hearts of mass shooters. Some have no compunction about attacking the teeth of tiger and going after trained and armed cops (Detroit police substation mass shooting, Dallas protest parade shooting of police). There are the few heroes that truly seem to make a clear difference such as Mark Wilson (firearms instructor, former gun range owner) who died in Tyler after shooting Arroyo and Vic Stacy (Early, Texas) who stopped a murderer who had a cop pinned down with rifle fire, potentially stopping a mass shooting. Of course, Mark Wilson died. In Houston, John Wilson tried to stop a mass shooter (Army veteran) who shot 7 including 2 responding officers, but before John could shoot, he was shot through both legs and was out of the fight, never firing a shot. Initially, cops thought John Wilson was a bad guy because he had a gun, but only later realized he was a citizen trying to do a good deed. Then there was Joseph Robert Wilcox. He tried to stop the CiCi's cop killers that entered Walmart in Las Vegas to make their last stand. When the male shouted to everyone to leave, Wilcox drew his concealed carry gun and was about to engage the male shooter when the female companion of the shooter, that Wilcox apparently didn't know was with the male shooter, shot Wilcox from behind, and killed him.
Getting back to the El Paso, and Styx's query, Walmart was filled to capacity with approximately 3000 shoppers, as many as 2/3 may have been from south of the border, leaving 1000 that were not. In Texas, 1 in 29 people (man, woman, and child, inclusive) has a permit to carry. Statistically speaking, there should have been a bunch of people in Walmart with permits to carry who didn't engage the shooter. Chris Grant, who apparently doesn't have a permit, was shopping with his mother who does. When the shooting started, he tried distracting the shooter by throwing things at the shooter and got to his mom to get her gun, which she was NOT CARRYING. Grant ended up getting shot and was attended to by another shopper at Walmart, a Customs and Border Patrol agent who was there off duty, who did NOT have her gun either. So we know that there were at least two folks at Walmart, one a LEO, that could have made a difference with their guns, but weren't carrying guns to make a difference with.
If people think that ordinary citizens are going to make some sort of big difference in stopping mass shootings with their legally carried guns (by permit, vehicle carry [in some states like Texas], or Constitutional carry), then these people need to 1) be carrying their guns, and 2) actually engage the shooter with them in a timely manner, knowing full well it may increase dramatically increase the risk to them. A LOT of people simply do not want to increase their personal risk in such situations. They carry guns to reduce their risk. Most will choose evacuation or standing and fighting if given any sort of choice at all. You can't blame them for that, right. However, these are issue at the crux of the problem of why non-LEO citizens aren't making huge differences in the scope or number of mass shootings, particularly in non-GFZs. Either they don't have their guns or they are not engaging. There is no official estimate, but based on CHL classes I helped RSO years ago, 80-90% of people with permits don't carry on a regular basis. Sure, maybe they carry some weekends, when they go hunting, when they go on long car trips, or occasionally when they have to go to the "bad part of town," but they don't carry all the time. Guns can be such a hassle. They are heavy, bulky, and you may have to change what you are wearing to fit with carrying a gun. Why would you unless you know it isn't safe? After all, what are the chances that you are going to need it, right? Like Chris Grant's mom or the CBP officer, they are just making a quick trip to Walmart to get a few things and don't feel the need to carry a gun...