What is the application? Do you need a lot of power from the LED? If so, you will need to use as much current as possible.

You will need about 20mA through the LED and you'll have about 1.7V across each LED, for a large range of currents. The supply voltage will be divided between all the components which are in series so, if you have a 6V (DC) supply and you want to have two LEDS, in series. They will take 3.4V, between them. You then need a resistor in series which will let through 20mA with the remaining 2.6V across it.

R = V/I so

R needs to be 1.3kΩ

If you are after as much light as possible then you will have as many series LEDs and a little series R as poss . The R introduces inefficiency, of course, but the LEDs, left to themselves, can take enough current to cook themselves -so they need something to control the current through them.

This link gives a lot of info.

http://www.theledlight.com/LED101.htmlUnless you have a big box of free LEDs, experiment with a large series R, particularly if you haven't all the data about the LEDs you are using and if you can't be sure of your sums.