Canon R5 C has a
45.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic X processor. On the other hand, Canon R6 has a
20.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic X processor.
Canon R5 C's sensor provides 25MP more than Canon R6'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 R5 C and R6 sensor size comparison.
Canon R5 C and Canon R6 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 R6 has 122% larger pixel area (43.00µm2 vs 19.30µm2) compared to Canon R5 C, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.