Re: How much do you really need?
I think you need to assess your particular situation in terms of defensive/offensive strategy. That will guide your tactics, etc. Your suppy of guns, ammo, and anything else will depend on that. Strategy consists basically of Ways, Means, and Ends. Don't just think about it. Write it down, revisit it and revise your plan as necessary.
If your only recourse is to bunker up, I guarantee you will lose at some point. As has been noted, "a cave is a grave".