By "combat" gun, I presume you mean the one you carry most often, and might have to use "the hard way". I keep over 10 magazines for each such gun. I keep two sets of three as carry units (hi-caps if they're available), and then two sets of three for training use. The training mags get a lot of use (including being dropped, kicked, etc.) during training courses, so I separate them from carry mags (also, I use 10-rounders for training, reserving the hi-caps for carry).
For a gun that isn't a first-line CCW piece, I relax, and go for five or six mags at most. For those I shoot relatively little, I'll drop to two or three mags.