LED's have various forward voltages largely dependent on color. So, if they were placed in series you have to have some voltage "left-over" for the resistor. If this was totally battery operated, then you have to have some "wiggle-room" or range that it will work. 8-12 V for instance. Each LED has a current that it operates at. Add them up.
A 12 V 0.5A battery makes little sense. A 12 V 500 mAh battery does. The model tells things about your battery. How long do you want the display to last? Drive by what?
All things change if these are "12 V" high intensity LEDs.
Your problem needs to be formulated better.