[Cliffradical]

I have 3 for my KJW Beretta, which seems to be enough for the 45 minute games at XT in Winnipeg (unless there are a lot of people and I need to use it for suppression).
5 is the most I've ever used in one game.
This of course depends mostly on your style of play, however, Ive seen people use less or more than that.
3 (2 plus what it comes with) is probably good number for an initial purchase- it will force you to learn without leaving you at a real disadvantage.
Also: invest in a good dump pouch. Watching your mags hit a concrete floors sucks.

Edit: Never ever ever lend your pistol to anyone. No matter how pleasant or experienced they are it just invites the wrath of the GBB God and bad things happen.