It's actually for one iron resource you get five swordsmen. Once one dies in the field, then you can build another to get your number of swordsmen back to five. If another civilization takes over the land with the iron resource, you do not lose your swordsmen, but cannot replace them when they die in battle (or by being disbanded). It means that the trading of strategic resources is now more interesting; before if you had two iron, the only use you had for the second instance was for trade, but now you need as much as you can get hold of....in the era that iron-based weapons are used that is!
