A mag pouch is a belt carrier that holds a spare magazine ready on your weak-hand side, so you can feed a reload fast without hunting for it. Together with the magazines themselves it counts as your allied equipment - the gear the belt carries alongside the holster - and it lives under the same position limits.
Those limits are the holster's limits. The magazine in a pouch can't stand more than 50 mm off the belt, and in Production, Production Optics, Standard and Classic the pouches sit behind the line of the hip bone, back on the weak side; Open and Revolver may carry them farther forward. Magazines belong in proper pouches rather than loose in a pocket, though one spare in a back trouser pocket is allowed as a reserve, and the placement is fixed for the whole match - you can't shift pouches from stage to stage.
How many you run is a simple calculation: look at the most round-heavy stage of the match, count the reloads it forces given your magazine capacity, and add one spare pouch on top. The reload itself - hand to pouch, magazine to gun - is trained the same way as the draw: at home, dry, off the exact belt you compete with.