Sigma fp L has a
61.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor . On the other hand, Canon 5DS has a
51.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6 processor.
Sigma fp L's sensor provides 10MP more than Canon 5DS'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 fp L and 5DS sensor size comparison.
Sigma fp L and Canon 5DS 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 Canon 5DS has 19% larger pixel area (17.17µm2 vs 14.34µm2) compared to Sigma fp L, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.