Sigma fp L has a
61.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor . On the other hand, Leica SL2-S has a
24.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor and features Maestro III processor.
Sigma fp L's sensor provides 37MP more than Leica SL2-S'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.
One other difference between these two cameras that is worth mentioning is that Leica SL2-S's sensor doesn't have an 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 fp L and SL2-S sensor size comparison.
Sigma fp L and Leica SL2-S 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 Leica SL2-S has 151% larger pixel area (36.00µm2 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.