A target of 1.000 rounds per caliber is a reasonable starting point however there are some variables to take into consideration. I think the .22 rim fire will be an important post SHTF caliber because it will be the primary game getter because most widely available game is small to medium size, in addition it makes less noise so attracts less attention, it is what you will be using to train new and occasional shooter the marksmanship skills they will need, and being the most widely owned firearm means it will be in demand and will serve as "currency" for trading goods and services. Therefore I think at least 5,000 rounds or one case makes more sense and would recommend buying two cases and when the first case runs out buy your next case so you always have between 5 and 10 thousand rounds on hand.
Another area you may want to increase stock is for whatever you will be using as a battle rifle and like wise anything you have multiple guns with the same chambers because you will likely be using these to arm family, friends and neighbors so your usage will likely be higher.
On the other hand your Big Bore gun for hunting Moose or Grizzley Bear you can probably stock less than a thousand because you will have fewer opportunities to use it because you don't want to shoot a Rabbit with a .416 Rigby, also you won't find a lot of people standing in line wanting to trade for it either.
Exactly this. I vave guns in over 60 calibers and I do not treat them the same... there are guns that I occasionally shoot because I like to ... but my plans don't hang on how much 9mm largo or .222 Remington I have. There are the important calibers and everything else. I make sure that I have enough of the important stuff and probably have 10x more 22lr than anything else. My list of important ones include. 22lr, 5.56, 7.62x39, 9mm, .45, and 12ga. I probably don't have a big game caliber at over 1000 rounds, but the combination of various calibers that will do the job is above that. Now shooting for pleasure after the tap is turned off... I don't think I'm quite at my comfort level for that yet.