ok this is how i remenber it.
your 5mm led are probably the basic ones.
white or blue led 5mm forward voltage should be no more than 3.3v and 20 mA for forward current.
Solution
- each 120 ohm resistor dissipates 48 mW
- the color code for 120 is brown red brown
- 1/4W resistors are fine for your application
- together, all resistors dissipate 192 mW
- together, the diodes dissipate 792 mW
- total power dissipated by the array is 984 mW
- the array draws current of 80 mA from the source.
- the 12v 500mA power source you have is plenty for your array.
- fill in the gap in the site i originally posted and you get a drawing graph of you array.