I ALWAYS have one pistol (90% of the time a .45 ACP) on me or right beside me. I very rarely (2-3 days per year) carry an extra handgun on me.
When I was on duty, if any of us carried a back up gun on us, we'd make sure we told who all we were working with that shift WHERE on us it was carried in case we went down somehow & the other officers needed extra guns / ammo. In 29 years 10 months, I personally never carried an extra or back up gun with me on duty. We went to semi-autos in 1985 and our cars had shotguns and then, as of 2001-2002, we had shotguns & AR's in each car. Plus, we all carried between 2-4 extra magazines on our belts for our duty weapons, which were all .40 cal.