Does it affect the sound, voltage, etc... to put two indicator leds (or 3, 4, ...) instead of one?
And what is the best, series or parallel?
No, it shouldn't affect the sound. Generally, the indicator LED is a separate circuit from the guitar effect. The only exception to this rule are for effects in which the LED is a flashing indicator, as in a tremolo or delay circuit. In those cases, you need to be a little bit more careful.
However, if you're tying an indicator LED in parallel with the rest of the circuit's power (usually how it is), you can experiment with multiple LEDs. LED's need a certain amount of power to run. Generally, it's lower than 9V, but more than 2V. We use a current limiting resistor (CLR) to lower the voltage crudely, so that the LED doesn't get burnt out by seeing too much power.
When you tie multiple LEDs together in series, there comes a point at which adding more LEDs actually brings the brightness of those LEDS down. It's at that point that I start adding new chains in parallel. You can have as many LEDs in parallel as your power supply can adequately handle (probably hundreds if not thousands). BUT, when they're in series, you have a limit that's set by the combination of the power needs of the LEDs, the voltage you're using, and the CLR.
Jacob
More current will be drawn in total than would happen with no LED. It'll be a very small addition, though, since it's an LED and not an incandescent bulb.
Thanks both, perfectly explained