if Vs-vf =0 do I need curent limiting resister?

CrossRoads:
That's 2V at 20mA. What is it at >20mA? Add a resistor to make sure it does not exceed.

Theoretically though to calculate the resistor required to ensure 20mA u use (vs-vf)/amps. So if vs-vf is zero then why do I need to put in a resistor?

If I do put in a resistor the LED's are not going to get 2v each. They will get less voltage will they not? Then they might not even come one?