The LED lamp consists of the LED and some circuitry that allows it to work at 12 volts. The wattage of your supply is volts x amps. 500 ma = 1/2 amp so you have 6 watts available. So doing the math and dividing 6 by .25 you could run 24 of the lamps. Now depending on the supply it might run hot if on continuously so you might have to drop off a few lamps.