extra ammo
As it is understood by us all that this is really a personal preference, I typically determine how much extra to carry based upon what the pistol holds, mag capacity plus the one in the chamber. I have been comfortable with just the basic amount in the pistol if that is 15 or more. When I carry a small frame 9mm with only 8 to 10 rounds in the pistol, I try to carry at least one more mag. One aspect of shoulder holsters that I think is great is the ability to have a double mag pouch on the off side, so when you grab your rig and throw it on, you automatically have a couple of extra mags with you without making a conscious effort to grab something else, in the case of the Steyr M or A1 or Walther P99, this gives you an incredible amount of ammo/firepower at hand if needed, 46 rounds for the Steyr (if using the new 15 round mags), 49 rounds for the Walther, that is a lot to carry ! Just my thoughts.