You will be throwing away a lot of power in the resistors. If this is an issue, use series parallel strings.
http://forum.allaboutcircuits.com/threads/wir...ltiple-leds-to-one-12v-dc-power-source.52820/You have to allow for the fact that a 12V battery is not 12V, if lead acid it starts out at around 14V, and drops to around 10V before it should be recharged. Other batteries behave in a similar fashion. So you have to design it with the maximum/minimum voltage in mind. An LEDs light output is related to the LED current, you need to check how much the brightness will change over the discharge cycle of the battery, if that is an issue.
If you need constant intensity, you need to use a SMPS, there are a number designed for this very purpose, driving series parallel strings.
To sort out resistor values, you need to look at the data sheets, figure out what the forward voltage drop is from a graph of voltage versus current (remember that current equates to light output, there will be a graph for that as well).
You need to check that with the calculated resistor value, it will work for the full span of the battery voltage, in terms of light output and power dissapation,
Cheers,
Richard
