You can use a combination.
Generally LED's are like 3.5-5 volts. So, 3 led's in series with the correct resistor is your best combination for a 12volt power source.
You CAN do 1 led for each, but it will have a different value than 3 in series, and you will have much more LED's.
I did mine with 3 leds off each resistor, got the toal voltage draw as close to 12volts and a resistor to go with each series link.
here are some led strips I made for a kegerator outa the back of my roomates truck. (Big skydiving party with like 8 kegs)