You could try five in series on 16V. If the supply sags to only 12V (as it should) with a load attached then it will drive four in series. Since the power supply is 1A and your lights are 500mA, you can use two strings of four (8 total) on it.
Before hooking them up, measure the current drawn by each light. You should only put lights in series with one another if they all use approximately the same current. For example, if you find four that draw 50mA each (3V), put them in series for 12V. If a light draws 100mA, it needs to be in a series string only with other lights which also measure about 100mA.