Separated into light and dark, it takes one unit of Light Ammo to fire a single Light Beam shot and one unit of Dark Ammo to fire a single Dark Beam shot with the Annihilator Beam taking one unit of each. Charged shots take five units of their respective ammunition to fire. To refill her ammo, Samus can destroy an object with one of the beams; the Light Beam gives Dark Ammo, the Dark Beam give Light Ammo, and the Annihilator Beam will give both. The refills come in units of five, ten, or thirty.
The original maximum of ammo she may hold is fifty units, but Samus can expand the amount she may hold with Beam Ammo Expansions, up to a maximum of 250.