It depends upon several factors
* Type of fire arm
* Cost of the magazine
* What I am using the firearm for
* How much the magazines are used
* Whether they are legal to manufacture any longer
First off let me state the obvious. Semi-Automatics are single shots without a magazine. So if a firearm is worth keeping and owning, then it’s also worth guarding against not having a functioning magazine for it. This is usually best accomplished through acquiring greater numbers of magazines or buying spare springs for the ones that you have or both for firearms that you count on always having. Additionally, I don’t feel that it necessarily makes sense to talk about the number of magazines per firearm. Sometimes you can talk about the number for a group of firearms that use the same magazine.
I have five 1911-A1 pistols. I shoot them a lot so I will tend to ware out magazines. I count on always having a functioning 1911-A1 for any contingency and magazines are relatively inexpensive. (Inexpensive is a relative term). For the first 1911-A1 I want 8-10 quality magazines; Wilson, McCormick, etc. but for the next 4 1911-A1s I don’t really need any more and usually you get some with the pistol. These are basic self defense firearms.
My AR15 is much like the 1911s in terms of always counting on it for a case of TSHTF. Magazines were more expensive for a while (but after the panic of 1999 they have come down closer to earth), I use them less, BUT THEY ARE NOW BANNED and CANNOT be MANUFACTURED so I have three AR15s and they have a stock of about 30 X 30 rounder and 20X 20 rounder. Hey, these may be all that my grandchildren can get.. These are basic self defense firearms.
One Browning High Power has less use, 6 Mec-Gar 13 round mags, 2 Browning factory mags which is what I use. The Mec-Gars are still wrapped. If mags were less, I would buy more 13 rounder. This is not a basic self defense firearm.
Beretta 92FS has only those high cap magazines that came with it. Those things are priced at nearly a hundred bucks. Too much for a pistol that is shot little and is NOT part of my basic self defense arsenal.
Makarov 9X18 has two mags. Not much use, they are cheap but if it stops running no big loss. I might buy a couple more sometime if the price is right.
I often see folks buy magazines when they don’t really need them now a days. This is probably because we all remember when you could buy all the high caps that you want for cheap. Now we all want to hoard now that they are illegal to manufacture.
Sorry, but your question is a complicated one and does not have a simple answer except as some one already said…..More is better.
PigPen