You should have 1000 cartridges handy for each caliber. After that, you can have as many as you want. The only consideration is if your house burns down. In which case, you don't want the bullets to burn when there are firefighters in the house trying to save your house.
So get a suitable cabinet or safe to store ammunition. Even so, there is a minimum for which you must shoot, and that is to have at least a thousand bullets per caliber, or better yet, per gun. Remember, ammunition is expensive and you may spend hundreds, if not thousands, of dollars on your stockpiles. There are several ways to decide how many rounds of ammunition you need to have on hand at any given time.
What guides the amount of ammunition I have in each caliber is based on the purpose of the firearm and whether it is my primary or secondary weapon. That's why it's so important to store ammunition in a sealed can and in a dry room at normal room temperature. Mark the date of your ammo and store it properly, and if you need bulk supplies, always look for ammo deals to get the most out of your money. It would be advisable to buy ammunition, at least 1 to 2 boxes, for calibers that you don't personally own or don't need.
Without proper care, it will only decrease the lifespan of your ammunition, thus wasting your own investment. Having no ammunition, or at least having very limited ammunition, would mean that your ability to defend yourself, your home and your family against the inevitable invaders and crowds is incredibly limited. In short, if you invest your money in two stockpiles of ammunition of 1000 shells per caliber, one battery to store it in the warehouse and the other to practice at the shooting range, you will be much more skilled (and lethal) with your gun and you won't have to worry about it running out in the event of a shortage or network crash situation. Trying to determine your ammunition needs for what-if situations involves a bit of methodology and a lot of guesswork.
You want to store as many of the best calibers of ammunition as possible, and you'll need to take good care of them to make them last a long time. And using ammo chests (incorrectly known as ammo cans) with really good seal rings and drop packs that DON'T touch the ammo is the best way. Instead, you need to buy an ammo container of some type that is specifically designed for ammo storage, and one of the best options is the. Basically, tell yourself that once a week, you'll walk into a sporting goods store and buy a box of ammo.
For a common gun, you need to store at least 1000 rounds of defensively loaded ammunition in order to use it in self-defense at home, and these 1000 rounds are very likely to last a long time if properly stored.