U mean 12 volts grouped together, right? 100mcd isn't that bright, especially when you compare it to a 3.5 volt 4000mcd led.
Can't really help u on what resisters to buy, but uh, don't go to radio shack for the led or resisters if you don't have em yet:
http://lsdiodes.home.comcast.net/
This is what that site says though
"The standard resistor for connecting LEDs to 12V sources: 470 Ohm". Just go into the resister section.