Pentax K-500 has a 16.0MP APS-C (23.7 x 15.7 mm ) sized CMOS sensor and features PRIME M processor. On the other hand, Sony A6400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor.
Sony A6400's sensor provides 8MP more than Pentax K-500'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 K-500 and A6400 sensor size comparison.
Pentax K-500 and Sony A6400 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Pentax K-500 and Sony A6400 sensors have been tested by DxO and the results show that A6400 has a better overall score of 83, 4 points higher compared to K-500's score of 79.