Why are there different number of propeller blades on the inboard and outboard engines? The Junkers G 38 has four propeller blades on the inboard but only two on the outboard engines.

What's the reason for this special arrangement and is there any benefit?
The inboard engines were of a different type, and more powerful than the outboard engines. Hence, the props had to absorb different power, so they were four-bladed for the inboard and two-bladed for the outboard engines.

Yo can see the specs of the airplane here
https://en.wikipedia.org/wiki/Junkers_G.38#Specifications_(G.38_1929)