For example, if I purchase an EAA Witness Match 9mm with a couple of 19 round magazines, how many more will I really need for the life of the gun? Do magazines really tend to wear out faster than say: firing pin, recoil spring, slide stop etc?
What about for competition - how many magazines do you typically use in one handgun competition? Rifle competition?
It will depend on how you use your firearms and what you use them for.
It is highly recommended that you have at least one spare mag per gun; the more the merrier of course.
If you shoot on the range a lot (say thousands of rounds a month), you may need a few magazines on hand. I won't say how many because that is up to you, but some say at least 5 to 10 mags per gun. Magazines wear out from use, (not from storage) so it is smart to have a few spares on hand when they malfunction or need to be serviced.
For competition-type use, you will need a minimum of at least 5 mags per firearm; I would say have a least 10mags or more if you are serious with IDPA/USPSA or 2-gun/3-gun matches etc.
For example, for 3-gun I have 5 mags for my Glock, and 10mags for each AR15. No mags for the pump shotgun of course.