They can be stored in various forms. The most common would be a block or brick of refined material: nanobots. Also, armor made of bots can be used as reserves, take a bulletproof vest, a chainmail or a platemail as examples. In this case, the user sacrifices defense for increased offense.
The most direct reserve for weapons would be a blade hanging on the side or attached to a limb or on the back. Operates roughly as a hidden blade in the second possibility.
The reserves are nanobots (separeted from or connected to the ring itself). If connected to the ring, they are capable of movement. Detached, they are rendered stiff until reattached.
