Nikon Z9 has a 46.0MP Full frame (35.9 x 23.9 mm ) sized Stacked CMOS sensor . On the other hand, Canon R3 has a 24.0MP Full frame (36 x 24 mm ) sized Stacked CMOS sensor and features Digic X processor.
Nikon Z9's sensor provides 22MP more than Canon R3'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 Z9 and R3 sensor size comparison.
Nikon Z9 and Canon R3 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.