Canon 1D X has a 18.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual Digic 5+ processor. On the other hand, Canon 5DS R has a 51.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6 processor. Canon 5DS R's sensor provides 33MP more than Canon 1D X'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 Canon 5DS R'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 1D X and 5DS R sensor size comparison.
Canon 1D X and Canon 5DS R 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 Canon 1D X has 180% larger pixel area compared to Canon 5DS R, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.