Hmmm, my first impression of the question was more of a strategic angle, and less of a tactical one, since it wasn't phrased "how many should I carry". I think maybe this was what Hard Ball had in mind too, but he can chime in for himself.
For anything I consider a primary carry gun (example: Beretta 92F, SIG P220, Glock 22) I think 6 are the minimum to have on hand (own). This provides spares in case of accidental damage, rotation "through the line up" (loaded), etc.
For secondary, deep CCW-type handguns (Beretta Bobcat, PPKS, etc.) that tend not be abused that much, and represent an alternative to a reload for the primary, I feel comfortable with owning three.