Hmmm....6 spare mags = 90 rounds + 16 loaded to go. Maybe another 500 rounds. I also believe in making every round count, that's why I spend time at the range.
Considering 9mm to be most readily available, you can always scavenge if you need to, including the BG you may have had to defend yourself from.
If I have to explain it, you probably won't understand it.