From Noah.org

LED color	If nominal	Vss
		5 V	12 V	15 V	24 V
Red	10 mA	270 Ω 1/8 W	1 kΩ 1/8 W	1.2 kΩ 1/4 W	2.2 kΩ 1/4 W
Green Blue Yellow Amber	20 mA	120 Ω 1/8 W	470 Ω 1/4 W	560 Ω 1/2 W	1 kΩ 1/2 W

series

White LEDs have a forward voltage drop of about 3.3V. The supply voltage should be about 1.5 times the forward voltage drop (total series voltage drop should be 2/3 supply voltage). So 18 white LEDs in series would need a 90V power supply.