It does vary widely. I’ve personally owned rifle magazines that have varied in capacity from 3 (in a bolt-action hunting rifle) to 35 (in a Ruger Mini-14 that I used to have). Most combat-style semi-auto rifles have at least 20-round mags, and most high-capacity combat semi-auto pistols have at least a dozen.
There is a real advantage to the high-capacity mags for combat purposes, or else they wouldn’t use them in the military and police. I will say that the laws against high-capacity magazines accomplish little, because as has been noted by many others, criminals don’t obey the law. There’s no high-technology required to make bootleg magazines; alternatively, there are plenty still available on the market.
On the contrary; he exercises his own self-control by not purchasing a firearm, rather than depending on the government to prohibit him from buying one. Same way with me and alcohol; I wouldn’t trust myself to know when to put the bottle down if I started (I’ve been a teetotaler all my life), but I rely on myself not to pick it up in the first place.
I’ll submit that the real question is whether or not the American people will become more and more acclimated to gun restrictions, and ultimately end up with the government disarming them. Then count the number of children who die in the purges to follow, as they did in the Nazi purges, the Stalin purges, the Mao purges, and a number of lesser purges in other nations. Count that against the number of children who have died in school and other shootings and in gun accidents over the same period of time on a per capita basis.