Whatever you have, basically. The bigger the weapon, the better, generally, though with some caveats. In particular, the triple-strike capability of serrated discs makes them more deadly against lightly-armored creatures than the larger giant axe blades, though against more heavily-armored creatures giant axe blades are better slashing weapons due to their improved ability to bypass armor.
Piercing weapons are generally somewhat undesirable in a weapon trap, as they appear to be more prone to jamming. Blunt weapons would be the best in terms of jamming resistance, but there's no blunt trap components. Since you're discussing dealing with elves and you've got iron, you'll probably want to load up traps with serrated discs. Once goblins start to be a problem, you'll want to either upgrade to steel or switch to warhammers or giant blades, as iron serrated discs won't penetrate their armor as effectively.
As far as how many weapons to put in each trap, it depends on how much area you need to cover and how many weapons you've got. It's better to have one or a few weapons in each trap in a mid-sized area than ten weapons in each trap in a very small area simply because then you'll at least wound most or every invader rather than killing just a few of them and then having the rest of the fresh enemies coming over your now-jammed traps. However, if you've got, say, 300 spare weapons/trap components sitting around, it's better to have a 3x10 area completely covered in 10-weapon weapon traps than a 3x100 area covered in 1-weapon traps since then you'll actually kill everything coming through instead of just wounding them all repeatedly.