I would say four as a minumum battery, assuming you are sticking close to home.

- A .22LR rifle for all the obvious reasons, OR very accurate (target grade, not compition) .22 revolver or autoloader ("race dragon"), OR pumped/lever charged .22 caliber air rifle (sneaky quiet, to, but not quite as much range).
- A 12ga shotgun, either pump or double (or a very unpicky auto), as it will be the most verstile thing you'll ever own.
- An auto-loading or pump action rifle in .243, .308 or .30-06 with a detachable box magazine (get some larger ones), or less preferably a .30-30 lever action.
- A good revolver, either .357 or .44 Mag, for defense, OR an autoloader in 9mm or .45ACP (.40S&W isn't that wide spread yet, still).

And if we want to talk TEOTWAKI, add in something .223 (preferably using mil-spec mags) and a flintlock (do you have to ask?).

Now, if I'm running for the hills, then the .308 and the race dragon. The .22 will kill anything under 50 pounds pretty consistantly with a good load and straight shooting, and the .308 covers everything up from the 50 pound range.