Amp-hours are current, irrelevant of voltage.But it makes no sense. What if i had 1S2P, that would mean 4000mAh pack ? Same as 6S2P ?

Power capacity is given as Watt-hours; that's amp-hours * average pack voltage (typically 3.7V per cell).

That works out that any series - parallel combination with the same number of cells (with all the same capacity) gives the same watt-hour capacity.

Use the configuration that gives the optimum voltage range for the device.

And remember the cell average voltage not cell numbers in calculations; 50W draw from a 1Sxx setup is an average of roughly 50 / 3.7 = 13.5A.