You have to play and find out for your self honestly. Everyone has a different style, every OP is different, ever role you may play is different and will require different amounts of ammo.
my team has a 10 mag minimum to carry on their person. I am a point man, usualy only fire in small bursts and semi and choose to carry 12-14. I also carry 3 100rd speedloaders on me for quick reloads if needed. Some events I'll only go through 3-5 mags...others ill go through my full loadout (including speedloaders) 3 times. It has nothing to do with the amount of people at the OP (been to games with 400ppl total and didnt even go through my rig once, likewise I was at a game with 40 last weekend and went through it 3 times) but more of what you are doing, how the game was set up, the terrain/structures and how well you can shoot your rifle.
Youll know what you need to carry with more experience...but as you can see everyone here carries a different loadout. some carry 5...I carry 14 at times (and im a light shooter for my team). Grab a box of MAG mids and you will be set for most of anything you will need, your rig/gear may be your limiting factor as it is mine. I cannot carry any more mags on my 1st or 2nd line gear because I am only 5'7 and 150lbs, the sized gear that fits me doesnt have ay more webbing left between utility pouches, hydro pouch and mag pouches.