20 3.2v - 3.8v LEDs in a line on a 12v circuit
To avoid having to burn too much power in a load resistor, you should put some of the LEDs in series.  For example:



Assume each LED drops 3.5V.  Then, the resistors will have (12V - (3.5V * 3)) = 1.5V across them.  If you want the current through the LEDs to be 10mA, then the resistor value should be (1.5V / 10mA) = 150 Ohms.

Each resistor in this case would be dissipating (i^2 * R) = (0.01 mA) * (150 Ohms) = 0.015 W, so any size resistor (1/8 W, 1/4 W, etc) would have no problem handling the power.

Note that this diagram has 21 LEDs, while you asked for 20.  To keep things simple and symmetric, it would be easiest to just install a 21st LED in the circuit and hide it, if you really only want 20 showing.