If the Imperials are anything like Power-Wrought swords from the Wheel of Time series it makes sense that they can be both 'rare' and 'common' at the same time. That is, if they're almost impossible to break or wear out, even if only a handful were created each year you'd be left with a lot of blades at the end with a very long lifespan just due to the fact you'd never lose any to attrition or 'wear and tear'