Nikon Z8 has a
46.0MP Full frame (35.9 x 23.9 mm ) sized Stacked CMOS sensor and features Expeed 7 processor. On the other hand, Nikon D6 has a
21.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor.
Nikon Z8's sensor provides 25MP more than Nikon D6'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 Z8 and D6 sensor size comparison.
Nikon Z8 and Nikon D6 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Nikon D6 has 119% larger pixel area (41.51µm2 vs 18.88µm2) compared to Nikon Z8, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.