Here is what worked for me for my 2 trips to The 'Stan. I am sure some have better techniques but this worked personally for me:
5 pistol mags; 1 in the gun, 3 mag pouch on the upper strap of my chest rig (opposite shooting shoulder; I'm a freak that shoots rifle lefty and pistol righty) the 5th I put in my E&E kit.
Found out that thigh rigs are cool but not very functional for general puropse (personal opinion here) i.e vehicle travel, running 1500 meters to attack an OBJ (that was a hoot!). I had a 4 mag pouch chest rig, each pouch held 3 mags. I'd stuff 3 of them with rifle mags and I used the last one to put my pistol in; it fit nice and rode well on the IBA, was up high for vehicle travel, balanced the rig well and was very handy. One mag in the gun, one mag on the stock and 3 extra in a side pouch. Be careful of stock pouches, make sure your charging handle will clear it especially on the M4 or equivelent. just my 2 cents
V/R
MTAB