Many warriors wielded Key like weapons, but for so many to have the potential to wield weapons that were so great in stature, its hard to imagine. They can't all be true Keyblades. I believe these "Keys" started out as normal weapons, but changed due to the reaction of the wielder's strength of heart. Suppose if Kingdoms such as Radiant Garden prepared their warriors for training, they were given sword or blades to fight in battles. Those who had stronger hearts were able to transform their basic weapons into Keyblades, or better yet a powered up weapons based on the warriors' strength of heart. So basically these weapons took on the shape and characteristics of the fighters' abilities. Those who could not perform such a feat were kicked off the unit or whatever. Perhaps other Kingdoms' Knights had the abilities to do that as well, although that's just assuming.