I can get 10 or 15 round magazines for my M&P 9, how many of what capacity does one typically carry for a stage at any match?

It depends what matches you are talking about and what divisions.  If you shoot IDPA or USPSA (Production, L10, SS) you would typically load to 10 although it is nice to have one that is larger so you can load it with 11 and be at 10+1 at the start without using a barny mag.  For Static and knockdown steel you are good to go with 10 as well.

I use 15 rounders all the time for competitions, even if I'm not loading to full capacity.

That's what I figured, get 15 round so I can easily do 10+1. How many do I need though? 1 in the gun plus 2 more on the belt? Or more?

5 is generally a good # to have. 

5 is generally a good # to have. 

 

Yep. 5 mags is the correct answer for 99% of the cases.

At least 5

7

Thanks guys!