How much current used in 20 LEDs circuit?
20 red leds (so 2.0 Vf at 20 mA) in series is 20 * 2.0 = 40 volts at 20 miliamps.  That's 40 * 0.02 = 0.8 Watts.

20 red leds in parallel is 20 * 0.02 = 0.4 Amps or 400 mA at 2 Volts. That's 2 * 0.4 = 0.8 Watts.

There is no difference in power used, just how you wire it. Current is shared in Series circuits, Voltage is shared in Parallel circuits.

This doesn't take into account resistors or current overhead. You need a current limiting resistor, or a constant current source. Arrange the leds as best suiting your power source or needs.