On my Bersa FireStorm, I only have 4 mags (all 8 round 380CC mags). I keep them all loaded. One in the weapon, one in my briefcase, and one in the glove compartment of each vehicle. My S&W Shield, I have 8 mags for. I keep 4 loaded (2 seven rounders and two 8 rounders), and four unloaded for practice. Same scenario with them as the Bersa Mags. My Glock 19 that I occasionally cary, I only have two of my 8 or 9 mags loaded. One 15 rounder in the weapon, and one in the outside pouch of a leg holster I use when hiking in the woods with the Glock.
When I was in the arms room, we kept 30 round M16 mags loaded. we issued them out daily and took them in, and used them as range ammo for quals twice a year. That rotated ammo and mags every 6 months. I remember reading in a professional LEO publication that it wears the spring out more to continuously load and unload them than it does to leave them loaded.