USPSA has a limit of no more than 32 rounds (required) on long field courses for their sanctioned matches.
That said with my Single Stack 1911, I usually carry 5 8rd mags plus one mag in the gun to start (49rds).
I'd gotten by just fine with 4 plus one for a over a year (41rds total) and never really needed the extra mag, but the first time you drop a mag or have one fly out of a pouch, you'll wish you had more
I think you should be just fine with the 42 that you have now, especially since with a revolver, you don't as often dump partial clips like us "bottom feeders" do with the semi-auto magazines.
