Starter motors generally have four carbon brushes. The name originates from one of their components: carbon. One of the first specialized distributors of starter brushes was the company Carbone Lorraine.
The brushes are attached via solder. There are two common mounting configurations: one where all four brushes are soldered to the brush holder, and another where the two negative brushes are on the brush holder while the other two are connected to the field coils. This latter configuration is found, for instance, on older Bosch starter motors.
You need the part number for your starter motor to find the right carbon brushes; you will find a wide range of brushes on wagendass.com.