When I started IDPA someone told me to have mags for matches and mags for training.
For guns used in IDPA with mags that can hold more than 10 rounds
, another idea might be: 1) mags for matches and training, and 2) mags for carry.
This lets you use 10-round mags for matches and training, as they're generally cheaper than hi-cap mags. Unless you're shooting a 1911 (8-round max), IDPA limits you to 10-rounds in a magazine.
For IDPA, it's a good idea to have 5-6 mags when you do a QUALIFIER MATCH so that you don't hold everyone up reloading 2-3 mags after each stage. Three mags simply aren't enough.
Keeping some mags loaded with carry ammo (premium HP) is a practical option, but keeping hi-caps or sub-compact mags
fully loaded can put extra stress on the mag springs and that may shorten their service life a bit. Wolff Springs recommends downloading those mags a round or two when stored to prolong spring life. For other mags (10-ronder 9mm mags in a full-size gun, or 7-round 1911 mags), it's not likely to be an issue.
You should shoot your carry mags (but not necessarily your carry ammo) from time to time, to confirm that the mags are functioning properly.