Outputs from these boards are constant-current and open drain. You can drive multiple LEDs in series. One resistor is used to set the current for each of the outputs, the constant current means that the LED brightness doesn't vary if the power supply dips.
We supply these with a 3.3K resistor for about 15mA per channel. But you can solder a thru-hole resistor over it if you'd like to change that value