Nikon Z6 has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor. On the other hand, Canon R6 has a 20.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic X processor.
Nikon Z6's sensor provides 5MP more than Canon R6'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.
One other difference between these two cameras that is worth mentioning is that Canon R6's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the Z6 and R6 sensor size comparison.
Nikon Z6 and Canon R6 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.