Nikon D750 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 processor. On the other hand, Pentax K-1 has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor . Pentax K-1's sensor provides 12MP more than Nikon D750'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 Pentax K-1'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 moire occurring in certain scenes.
Below you can see the D750 and K-1 sensor size comparison.
Nikon D750 and Pentax K-1 have 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 D750 has 49% larger pixel area compared to Pentax K-1, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.