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You need to know the max current of the LED and the rated voltage for the LED.
9V - LED Voltage / current of the LED = Resistor value.
As a personal rule of thumb I prefer to stay slightly under the max rating for LED's to increase their lifespan.
As an example:
Voltage for the LED assumed to be 2.3V and a max current of 15mA:
(9V - 2.3V) / 0.015A = 447Ohm
At 470Ohm it will be 14.25mA
At 500Ohm for the resistor the current will be reduced to 13.4mA
Next higher standard value would be 560Ohm and 12mA
If you are running up to 100 of these led's why not wire them in Series?
Wire a group of three LED's in series with a 150-Ohm resistor !
I would assume they will work plenty bright at 15mA, so I would use that figure, then you need to know how much voltage drop across the LED is to figure out the perfect resistor. You can do that with a potentiometer and a few multimeters, (to measure voltage and current simultaneously) or just refer to the datasheet for the numbers.
You can always just use a 1K resistor. That will be fine.
In this video, I have included a guide about 2/3rds the way through a guide on calculating the exact resistor to use to get the most power out of the LED without overdriving it.
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