As someone who really started in the hobby/sport in November of 2010 (prior to that, I just had the usual "fun days" at the range with friends, and never owned any firearms, myself: before purchasing my first firearm in October of 2010, I literally last touched a firearm in maybe 2004 or 2005), and who actively engages in training - and will start competing likely this summer - you can likely guess my answer to the above.
Here's why:
Having gone through - and trained during - "The Craze" of 2012 (typically known as the back half of the 2018-13 shortage, which, when you count .22LR, actually extended to some time in 2016), I currently maintain a baseline of about 10K rounds of 9x19 and .223/5.56, my two most commonly used calibers, with around 1,000 rounds of defensive ammo for each.
That sounds like a decent cache?
Not really, considering that the typical one-day (8-hour) training class usually goes through 500 to 1,000 rounds. To put this in-perspective, the last class I attended was a 3-hour seminar, and I went through close to 600 rounds. One season's worth of diligent training can literally wipe out my entire stock of range-fodder, and similarly, the yearly vetting of my defensive weapons' function/zero and magazines takes out an average of 100 rounds per gun.
So, each year, I add more, while trying to maintain that base because I don't want to go through what I had to go through during "The Craze" again.
However, it's more complicated that that: I'm also a first-generation immigrant.
With both of my parents being non-shooters, this means that it really has come to me to begin a 2A-legacy for my daughter...and maybe even her children. This thought is honestly never far from my mind, in thinking of what kind of firearms I would have had access to had my parents been shooters, and what they may have had to pass-down to me. To an extent, this is what also drives what I buy where it comes to firearms and gear: "buy once, cry once" is not only applicable to the durability/reliability of the items that I want, but also has implications on being able to hand these items down to the generations yet to come.
And really, with that, both ammo - and even magazines - becomes a part of the equation. Legalities of ownership aside, I figure I can at least cover my daughter on the question of what would happen if ammo can no longer be bulk purchased (California, anyone?) or if standard-capacity magazines can only be grandfathered-in? Or, for that matter, what happens when something is simply no longer being sold by the manufacturer any more - look on XDTalk, and you'll see WTB ads for the 13-round "flush" XDm 3.8 Compact magazines. As most of you know, I use this gun for my EDC: I'm pretty glad I got a bunch of those magazines, when I could......
.22LR and the other odd calibers? It really depends. While my daughter and I both plink with .22LR, our range-time together is infrequent, so my cache (somewhere in the 25K rounds range) has remained pretty steady. On the other hand, I'm only at about 500/300 rounds of range/defensive .357-SIG and 12-gauge, while my stock of .22WMR is a complete mix. There's also probably only somewhere around a 100/50 round split of .38-Special, with maybe less than a box (each, range and defensive) of .357-Magnum.
So, like
@10mmLife , I actually try to not only replenish what I've shot, but also add just a little bit more to it over time.
Buy it while the buying is good!