Power output equivalent of 18V battery
Series

No, a battery comprised of a series of cells that each can supply 20 A will only supply 20 A. The same current runs through all cells. A 10-cell 18 V battery uses cells that each produce only 1.8 V. You connect them in series to produce a higher voltage useful for driving an electric motor.

This gives a power output of 18 x 20 = 360 Watts.



Parallel

If the cells were reconnected in parallel (which is usually unsafe), You could produce 10 x 20 = 200 Amps but only at 1.8 V.

The power output would be 10 x 20 x 1.8 = 360 Watts. The same. However you would need much thicker wires to carry the current and/or the power losses in the wires would be higher.



18650

The 18650 is a 3.7 V 2200 mAh cell. To produce a 18 V, 2200 mAh battery you would connect five of them in series. To produce a 4 Ah (4000 mAh) battery you would use ten cells in a combination of parallel and series.

According to one data sheet The maximum continuous discharge is 2.2 A (2200 mA) not 20A. In that case, drawing 20A for more than a few seconds will overheat the cell. The cell has overcurrent detection at 4 - 7 A.