I'd suggest 5 mags for USPSA Production. Six is better, in case you break a base-plate on the 2nd stage!!!
Most stages will be less than 30 rounds, but you might run into a "Texas Star", or a 40yd plate rack, that requires a few more shots... Or heaven forbid you get a jam, you'll want to clear, and reload a full mag.
Production limits you to 10 rounds per mag, plus one in the chamber to start.