For so-called 'milsim's', I suppose that the correct answer would be whatever is period/role/loading correct, beyond raw-ammo count limits imposed by the game.
A pistol is considered a secondary weapon/back-up weapon, not the primary. It's there to fall back to in case of a jam/stoppage/empty of ammo, that can't be dealt with immediately, in the face of imminent threat, or in cases of extreme close-range engagement.
In theory, you don't need to be carrying the same quantities of magazines that you would be, for your primary.
In so-called 'milsim's', most people stick to one secondary. I have seen some people carrying two secondaries ie. one compact SIG P229, and a full-frame Beretta M9, or a compact and a full-frame 1911 carried in different location on the body - both examples obviously using common ammo types, so keeping it semi-accurate to the need for real-world standardizing.
I run 3 mags, per secondary .. one in the gun, two mags to fall back to, whether skirmish, or 'milsim'ish'.
Last edited by HackD; October 18th, 2014 at 18:57..