this would depend on a few things, for example does the LED have a resistor built in or not, what is the voltage of the LED, what is the voltage of the batteries, are you connecting the LED's singular or more than one LED in a circuit, i have built myself an Excel worksheet that works the required resistor out for me.
here is an example:
connecting a single LED
LED in Volt U=3,50 V
LED in mA I=18mA
Battery in V U=6,00 V
Resistor in Ohm R=138,89
Power loss through resistor P=0,05 W
not forgetting that LED are Pole sensitive, the long leg is the positive leg...