The 3x3x3 Rule....
In general, I follow what I call the 3x3x3 Rule;
That most ARMED CITIZEN(not armed security, law enforcement, corrections, etc) or concealed license holder(s) will have a lethal force incident with 3 rounds fired total, at 3 feet or less, in approx 3 seconds.
Most violent attacks are over in a few seconds and either the subject will flee or be seriously wounded/killed. This does not mean you(as a license holder or gun owner) should only carry a 5-6 shot revolver or 15/16 round semi auto pistol w/o any spare ammunition.
You do not need 33rd Glock magazines or 4 magazine holders like the Galco add-on or Tuff Products carrier either.
I'd feel more prepared knowing I could do a "tactical reload" or recharge my carry gun in a critical incident(a smart tactic, IMO).
Also, don't think that all street thugs only have one weapon or no spare rounds/back-up. A recent SWAT magazine had an article about a LE officer who routinely found criminals with multiple weapons & spare magazines, rounds.
The smart & practical move for an armed citizen or license holder is to "carry enough gun" and pack at least 1 speed strip/magazine.
Clyde