Leica M-Monochrom has a
18.0MP Full frame (36 x 24 mm ) sized CCD sensor . On the other hand, Leica M11 has a
60.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor and features Maestro III processor.
Leica M11's sensor provides 42MP more than Leica M-Monochrom'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 M-Monochrom and M11 sensor size comparison.
Leica M-Monochrom and Leica M11 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 M-Monochrom has 233% larger pixel area compared to Leica M11, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.