Nikon Z50 has a
21.0MP APS-C (23.5 x 15.7 mm ) sized BSI-CMOS sensor and features Expeed 6 processor. On the other hand, Pentax K-m has a
10.0MP APS-C (23.5 x 15.7 mm ) sized CCD sensor .
Nikon Z50's sensor provides 11MP more than Pentax K-m'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.
Another difference between these two cameras is that Nikon Z50's sensor lacks 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 moiré occurring in certain scenes.
Below you can see the Z50 and K-m sensor size comparison.
Nikon Z50 and Pentax K-m 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 Pentax K-m has 105% larger pixel area (36.75µm2 vs 17.85µm2) compared to Nikon Z50, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.