How do I power 100 LEDs in series with low voltage?
If you want to power all of the LEDs in series, you only need a single current-limiting resistor for the entire string. However, your power supply voltage must be at least the sum of all of the LEDs' forward voltages. If that's not convenient for your application, you should divide them up into separate strings and connect the strings in parallel.  Importantly, each string should have it's own current-limiting resistor.