There's no national data base for all police shootings, let alone those involving armed citizens. It's generally well above 2.3 shots, though.
You've articulated very well the reasons why, today, more ammo is likely to be needed than 50 years ago, once the shooting starts.
For longer than I've been alive, the national standard for cops has been a fully loaded gun and two full reloads on the person when in uniform. When I started, that was six in the service revolver and twelve more cartridges on the belt in loops or pouches. Today, with autos, that same standard gives you 16 rounds in, say, a department issue Glock 22 and another 30 on the belt, and I don't hear any cops complaining about having to carry too much ammo. You'll be facing the same people they will, and likely without backup personnel on your side. Carrying spare ammo seems to be simply common sense.