I have two mag carriers to accompany each holster -- two pocket Remora pouches to go with my Remora holsters, and two leather OWB pouches to go with each of my UGB Regulator holsters. I have one nylon holster with a mag pouch integrated into the front -- a nice design, but I've yet to use that holster.
I typically do not carry a spare mag -- shame on me; but, when I do I wear one UGB OWB pouch on my offhand side, or carry one Remora pouch IWB or in my front pants pocket on my offhand side.
One design I'd like to try is a horizontal OWB pouch, as that looks to be the most comfortable -- nothing to poke you.