When I used to CCW back in the good old days.... I always carried 2 extra magazines for whatever gun I had on me, and if I had 2 guns, extra ammo for the second one too!
I went to work one night with my G17, 18 rounds in the gun, 2 extra mags with 15 rounds each, with my Model 13 S&W as a backup 6 in the gun, 12 in speed loaders, and for good measure my Raven .25ACP 7 in the gun, 14 in magazines.
You can never be too rich, too thin, or too armed