Nikon Z5 has a
24.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor. On the other hand, Nikon Z8 has a
46.0MP Full frame (35.9 x 23.9 mm ) sized Stacked CMOS sensor and features Expeed 7 processor.
Nikon Z8's sensor provides 22MP more than Nikon Z5'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 Z5 and Z8 sensor size comparison.
Nikon Z5 and Nikon Z8 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 Z5 has 88% larger pixel area compared to Nikon Z8, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.