The doping input in QuantumATK is actually given in number of electrons, a dimensionless number. The volume or area is just used as a scaling factor if you want to give the number in cm-3 or cm-2.
So for example, if you want a doping concentration of 1e12 cm-2, then multiply by the area of the sheet (i.e. the area of the AB plane in the unitcell, if C is perpendicular to the sheet) and you have the compensation charge you need to input into the calculation.
