However, I have realized that five is the ideal number for me. That way, I can keep two loaded with carry ammo, and have three for the range, training, and IDPA use.
+1. FWIW I've found that, for me, taking more than 3 mags per pistol to the range is overkill. It becomes more work to keep track of my spares than to reload the ones I've used. Leaving a $35 mag at the range sucks, done it once, don't want to repeat it.