The nice thing about ammo is, as long as you store it properly, it doesn't really go bad. So, the idea that you can "never have enough" isn't really a bad one as long as you've got the proper storage space. Worst comes to worst you can often resell it, trade it, or just shoot it.

With that said, I've read varying opinions on how much ammo to keep.

Personally, I used to shoot in .22 prone competitions, where I would go through around 1,000 rounds of .22lr a week between practice and matches. Therefore, I would try to keep at least a case on hand (5,000 rounds) just in case, for some reason, I didn't get my weekly supply of ammo. For me, that was the bare minimum. However, someone else might not use that much in a year. So the amount varies per person based on current usage and planned usage.

You also have to examine local laws, some places have limits on the amount of ammo you can store without a Haz-mat permit of some sort. Same with your insurance company, sometimes they put limits on ammo or gunpowder storage.

With all that in mind, here is the reasoning for the amount of ammunition I keep on hand:

If I remember correctly the standard load of rifle ammunition for the US military is 210 rounds of ammunition split among (7) 30 rounds magazines, and that is supposed to last an average soldier 48 hours under normal fighting conditions. (Whatever "average" and "normal" are? ) However, many do carry more than that.

Now, the government also says we should stock at least three days worth of food and water.

Therefore, going by those numbers, I figure with every three day supply of food and water, I also like to stock around 500 rounds of ammunition with each firearm, as that would probably prepare me for worst case scenario (military action), have enough for a much less hostile scenario (hunting/basic protection), while not being what some would consider an unreasonable (obscene?) or totally immobile amount of ammunition.

Now, that number doesn't take into account how many capable shooters you have or what kind/how many firearms you have. So you may want to have more or less depending on firearm type and amount, or more or less based upon how many people you expect. It's only my somewhat arbitrary baseline.

For example, you might not want to stock 500 rounds of .375 H&H for your grandfather's antique doubles rifle, as that would cost over $2,000 and it's probably not a gun you're going to shoot that much, especially if you live in "the lower 48." But, you might want to stock more than 500 rounds of .22LR, because .22LR is so small, useful, popular, and easy to shoot. You have to take those things into account.

Long story short, there isn't a perfect set amount. IMO, the best thing you can do is come up with a reasonable baseline from other sources and then modify it based on your own individual needs.