All you want and can store is the obvious answer!
Really, you can't have too much ammo. But you CAN have too little.
I want at least a thousand rounds on hand for EACH caliber I shoot. I wouldn't object to five thousand or even several times that.
If the S truly looked like it was going to hit the fan and "The Postman" looked like a reasonable prediction of the future (or worse yet, "Mad Max") then I'd quietly start buying every case of ammo I could find and also all the suitable reloading supplies I could get as well.
I don't think having a supply of 50,000 rounds would make me invincible, but it WOULD be handy to have if only to provide my most trusted friends and relatives with some if they needed it.
As for how much is enough in actual battle conditions, ask yourself how good a shot you are as compared to your ability to disappear. If you stand out like a neon sign in the desert at night, you won't need more than two shots before you're a statistic. But if you can camo out with the best of them and are a kick-ass shot, you can probably make good use of all the ammo you can easily transport, which should be between 300 and a thousand rounds, at a guess.
Personally, I keep at least 20 30 round USGI mags loaded at all times. Not for SHTF preparedness, but just because when I go to the range I prefer to grab a loaded mag instead of fiddle with reloading an empty one.
CJ