Fujifilm X-T3 has a
26.0MP APS-C (23.5 x 15.6 mm ) sized X-Trans CMOS 4 sensor and features X-Processor 4 processor. On the other hand, Ricoh GXR A12 50mm has a
12.0MP APS-C (23.6 x 15.7 mm ) sized CMOS sensor and features GR engine III processor.
Fujifilm X-T3's sensor provides 14MP more than Ricoh GXR A12 50mm'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 Fujifilm X-T3'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 moiré occurring in certain scenes.
Below you can see the X-T3 and GXR A12 50mm F2.5 Macro sensor size comparison.
Fujifilm X-T3 and Ricoh GXR A12 50mm have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.