1. Personal preference. If you are worried about a magazine ban (with grandfathering), more is better. Otherwise, I'd say however many you would want to use at once times 1.5 or x2, just to cover potential damage. Frankly, there are enough 10-pack Pmag deals out there that one of those would be a logical choice.
2. Again, personal preference. If you would use it for defensive purposes, I'd suggest at least one. Having it in the safe in condition 3 might not be a bad idea, so long as you respect the status of the firearm during handling. Running out the door/back to the bedroom while shoving loose rounds in a mag is not a great idea, despite the guy in TX's results against a mass shooter. If you fear zombie uprisings or just don't like loading mags before heading to the range...more, I guess.