Canon T8i has a 24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 8 processor. On the other hand, Canon R has a 30.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features DIGIC 8 processor.
Canon R's sensor provides 6MP more than Canon T8i'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 T8i and EOS R sensor size comparison.
As seen above, Canon R has a 2.6x Larger sensor area than Canon T8i. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.