But that's not the same number as the amount you should buy. Given the non-zero chance that you go out to resupply and it's sold out, you need to buy enough so

But that's not the same number as the amount you should buy. Given the non-zero chance that you go out to resupply and it's sold out, you need to buy enough so that you can last until the next restock. For most people, just going out to buy it sooner than they normally would is likely enough to give them leeway. However, that still means they will have more stashed than they normally would (since they don't allow themselves to get as low).