Okay, I know the obvious answer is “You can never have too many Nerf guns”, but beside that point, it's something that runs through nearly every nerfer's head once they collect more than a couple – which ones do you pick for tomorrow's war, and how many?
Ultimately, it will depend on a number of factors. First is what kind of game and where. Inside or outside? Long range or close? HvZ or free-for-all? Assassins or Capture the Flag? Will the games be played in short rounds (so you can re-fill clips), or does the game go all day?
This should dictate what your essential, first pick is – the blaster you own which is most suited to the role.
Further choices will depend on what your first pick is. You might pick something to cover the primary's weaknesses, or carry something similar so you can keep firing when it runs out.
In principle, you can keep picking blasters until you're carrying everything you have.
Then consider how you're carrying it all. There are a number of options, ranging from the Nerf-produced bandoleer kit and vest kit, to army-issue load-bearing vests, home-made holsters and the like. I've even seen crazy schemes where someone had six Reflex pistols velcroed to a dart tag vest. If you really try, it's more than possible to carry ten blasters. In practice, though, I rarely see anyone carry more than two or three. Why? Despite (mostly) being lightweight, Nerf guns are bulky. The KISS (Keep It Simple, Stupid) principle also comes into effect. Just remember – the more foam weaponry you carry, the less space to carry ammo.
Then there's your “spares”. What if someone else needs to borrow one? Maybe the game changes as the day goes on? Fortunately, you don't have to carry these around during the match, so you've got a bit more flexibility with their type and size. The only limit on your spares will be how many you're willing to load in your car or can fit in a bag.
But I've gone on enough – how many do you guys take to wars, and how do you carry them?