I think most people die after using a maximum of 2 magazines. If there are an average of 8 deaths per player, 8x24x2 = 192 magazines.
If you use one of the reload-on-respawn scripts, you won't need to put so many magazines in. My own method is respawning with the weapon and then crates full of extra optional weapons. There would be, say 20 rifles and 120 magazines for that rifle type in the crate.
I think 200 mags will do just fine, if it seems to come up short try going up to 400.