I think everyone should do as they wish and carry as many rounds as they can possible carry. But I have trouble seeing any circumstance within the realm of reasonable possibility where you're going to need more than a few rounds.
If you're running out to your vehicle to get more ammo it means you've disengaged from the immediate situation and are planning on returning to the fight, which negates your argument of "self-defense". Also you're assumedly returning to a situation where there's going to be a police response and the last thing they'd probably want is another unknown person wandering around with a weapon while they're looking for a bad guy.
"Our contract called for 16 cases of rifles and ammunition for $10,000 dollars, not a machine gun...........That is our present to the General"-Pike Bishop