And for two LEDs, just connect a second one in parallel. The resistor is called a "current limiting" resistor. Without it, 9 volts will charge through your LED and blow it instantly. You probably don't need a second resistor, as the first one serves a common purpose. Adjusting its value will control how much current flows through the LED, and how bright it will be. Then again, if you want to dial in the brightness for each LED (for instance, different colors), you may want an individual resistors for each LED.