Sony A9 has a
24.0MP Full frame (35.6 x 23.8 mm ) sized Stacked BSI-CMOS sensor and features BIONZ X processor. On the other hand, Leica M-Monochrom has a
18.0MP Full frame (36 x 24 mm ) sized CCD sensor .
Sony A9's sensor provides 6MP 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 A9 and M-Monochrom sensor size comparison.
Sony A9 and Leica M-Monochrom 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.