I'm certainly no expert and have never been involved in a shooting. However, JMHO I think a lot of this high capacity stuff is fantasy from wannabes and tacticool operators who think all out war is gonna break out in the streets at any minute. Think about it. When was the last time you actually heard of a gunfight going more than maybe 3 or 4 rounds?
Now gangbangers routinely empty 16 rd magazines at each other. How many hits do they get?
I remember here in Memphis maybe 15-20 years ago, a guy was attacked at a local mall. Victim pulled his own gun and shot the BG 16 times emptying his mag! Pumping about half into the BG after he was already down. Yep, the victim paniced and just kept shooting. It was ruled a good shoot eventually, but for a while there it was touch and go just due the number of rounds fired. The defender was almost toast by his own doing.
I've been carrying a gun for about 30 years now. The vast majority of the time my carry gun held between 5 and 8 rounds. I do carry a reload, but I've never felt under gunned.
Again JMHO, but I think if you actually were confronted by 20 BGs, once that first shot was fired, they'd scatter like a covey of quail.