What gauge wire for 24V, 72W, 8m extension RGBW?
The issue is the current times the resistance per meter. 2.1 / n ohms = how much voltage will drop across it. Say 0.1 ohms per meter, 2.1 Amps / 0.8 ohms = 2.62 Volts. That section will drop the voltage your LEDs see to 21.38 volts.

Since your common carries the current for all of your channels, you must use the sum of all the current, for this section. Let's say 8 amps. That's 8 / 0.8 = 10 volts. So combined, 24 - 10  - 2.62 = 11.38 volts left for your LEDs. Of course the actual drop will be different because your LEDs will not have enough voltage to turn on so the current is less and the wires will not drop the same amount of voltage.

So grab the resistance per meter for a few different wire sizes and calculate what would be an acceptable voltage drop for your setup.