I tend to see most shooters with 6 mags in total when heading to the line to shoot. 4 on the belt and 2 in pockets, 1 which is the feeder and the other being the mag they will start the COF with.
As for total number of mags, it depends. I have about 15 mags for my G17, I load em before the match and then dont usually have to load anymore while at the range. Also, not having to reuse a mag can be handy if your shooting in bad weather and your mags are getting dropped in water and or mud, or perhaps its a dry range and the dust is getting inside the mags when dropped.