I've shot IPSC a few times in production and I had three mags on my belt and started with one in the gun. I put another mag in my back pocket as a stage saver.
Its all really dependent on how long the courses of fire are at your range. If you have small bays and everyone turns and burns quickly you can get by with 3 mags + whatever's in the gun. If things get elaborate or SHTF like, I've seen guys pack THREE of the Fobus Dual Mag Holders on their body and still run out in Production.