Many LED's drop 1.2 volts across them so you have 12 minus 1.2 = 10.8 Volts. Assuming a current of 15 ma, you might try a 720 Ohm resistor. Not knowing the exact LED's you have, no one can say exactly. If you want to be on the safer side, start with a 1K resistor. Somewhere between 10 and 20 ma should provide sufficient brightness. Disclaimer: If you burn them up, I'm not responsible.