Here in Arizona we frequently shoot a stage with 30 pistol rounds. That means 6 mags. If you have a problem of some kind, like dropping a mag while moving, you may need another mag. So I always take 7 to the loading table. One for the pistol and 6 to use as needed. You can pick up a dropped mag, but I don't if it's on the ground. If dropped on a shelf, you bet I'll pick it up.
Anyhow, I recommend having at least 7. You can carry the 7th one to the loading table in a pocket and have 6 in pouches.
Hope that helps.