The resistor you need for the LED is relatively simple maths. You need to know the current draw and voltage of the LED. If you don't have this available assume 1.6v and 20mA (0.02A) and work from there.

If powering the LED from 12v you need to drop the voltage by (12v - 1.6v) = 10.4v at 20mA.

Ohms law says that V = IR, or R = V / I. Resistance you need is therefore 10.4 / 0.02, or 520 Ohms. Lots of people just use the readily available 470 Ohm resistor and accept that they're going to shorten the life of the LED a bit.

The same maths works with multiple LEDs. If putting LEDs in parallel they each need their own resistor (i.e. 520 ohm each). If putting LEDs in series you can power them all from one resistor. The current remains the same but the voltage will change...

Four LEDs, each at 1.6v, would be 6.4v. Voltage drop across the resistor would be (12v - 6.4v) = 5.6v.

The resistor can be calculated as:

R = V / I, so R = 5.6 / 0.02 = 280 ohms.

Note that you must have a resistor, even if you have enough LEDs to meet your battery voltage. The resistor serves to drop the supply voltage if the LEDs try to draw more current, thereby self-regulating the current through the LED and avoiding the LED blowing up.