I have quite a few guns, I won't go through the entire list (don't need a grocery list for thieves or other miscreants) but I have the following calibers:
Shotgun:
-12ga
Handguns:
-.22lr, .380, 9mm, .40S&W, .45ACP, .38spl, .357mag, and .41mag
Rifles:
-.22lr, .45LC, .223, 7.62x39, and 30-30
So, like you I have several options. My view is that it is better to pick a couple guns for your SHTF guns (with 2 or 3 possible scenarios in mind so you are prepared for all options) and stock up in those calibers. Otherwise, only stock a couple boxes to a couple hundred rounds (whatever you usually will use in one range session). It isn't cost or space efficient to do otherwise. Also, it is better to already know what to grab in a situation instead of having to think it out and plan then.
For SHTF scenarios, the most likely is a temporary loss of civil authority like a riot or a natural disaster. These situations call for what you normally would use for home defense anyway plus maybe a quick loading, medium range rifle (.223, 7.62x39, lever rifle in 30-30 or a magnum revolver caliber, etc.).
For the theoretical but not realistic possibilities like an invasion or complete breakdown of society it depends upon your mindset. I don't like the idea of going on the offensive against large groups of trained soldiers or mobs of people. I prefer the idea of bunkering down and using defensive armament, or maybe sniping at stray enemies from time to time. For the defensive armament your needs aren't much different from the more likely scenarios but with more ammo (since you may not be able to replenish), plus maybe a .22 for food getting. For sniping you'll want a .22lr (quiet) for short range and/or a good scoped bolt or semi-auto rifle for distances. IF you plan to go on the offensive you won't last long, but you'll probably want a mag fed "assault rifle" or battle rifle for while you are still alive.
In my case I have long stocked up on 9mm for my handgun "SHTF" caliber. However since for economy sake I stock FMJ for long-term storage (rarely more than 100 rounds of good JHP due to costs) I'm thinking I'm going to switch to .40S&W or .45ACP. With JHP 9mm is very capable, but with FMJ the larger calibers are much better. I also stock .22lr since it is cheap and I have both a rifle and handgun in that caliber and .38spl for my snubs. I also have components to reload about 500 .357mag, plus I usually have a box or two, in case I'm running to the woods (I use a hand press so if I have access to a car I can bring my reloading gear). I generally plan to take 2 guns (a service pistol and a snub) if I need to bug out, but I'll take my 6" S&W 586 if I'm bugging out to the mountains/woods in western MD, WV, western PA or western VA.
For rifles I stock 7.62x39 (it is cheap, good for short/medium range defense, and my SKS is very hardy), occasionally .223 (it is cheap, until I get another bolt rifle the closest I have to a "reach out and touch someone" gun is my AR, and it is great for short range/medium range defense), and occasionally 30-30 (a lever rifle is much more PC looking and thus more likely to be ignored by authorities yet still a good short/medium range defensive gun that can be loaded/topped off on the run). These three calibers give me a good deal of versatility in determining how to respond to a situation. Of course, I also have a few thousand rounds of .22lr around all the time so my Marlin 60 is an option as well.
That is in addition to what I usually have around for home/self defense (25 rounds of 12ga low recoil 00 buck, 20-40 rounds of 125gr 9mm JHP, 20-40 rounds of 165 or 180gr .40S&W JHP, 20 rounds of 230gr of .45ACP JHP, 20 rounds of 125gr .38+P JHP) and recreation (generally a box or two of FMJ minimum for each caliber I own).