It depends on the scenario. Lets say Meltdown of Society, or really rough times.
Since my main handgun would be the .357 for versatility, I would elect to have no less then 1000 rds. 4-500 of those being .38spl since I also have some .38s. The reason being is that .357 can be used on white tail and the .38s on smaller game. The rest of the handguns would only get fed a couple of hundred. This is provided I have powder, bullets and primers for my wheel guns.
As for rifles. I view 1000-1500 per caliber for the Aks/SKS or AR15 if you go that route. And at least 400 for your bolt guns. Currently the only thing I'm lacking on is ammo for my AK/SKS. I need about another 1000 rds case to get up to speed on that.
Now if it is invasion of the US by some other nation, I dont think I could put a limit on it. I would like to have several ammo dumps in each neighboring town as well as a few here in mine.