Nikon D810A has a
36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features EXPEED 4 processor. On the other hand, Nikon Df has a
16.0MP Full frame (36 x 23.9 mm ) sized CMOS sensor and features Expeed 3 processor.
Nikon D810A's sensor provides 20MP more than Nikon Df's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the D810A and Df sensor size comparison.
Nikon D810A and Nikon Df have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.