How many bullets does a MG42 hold?

First, the term is cartridge. Bullet is the part of the cartridge that comes out of the barrel.

Second, the MG42 holds NO cartridges. It is a belt fed machinegun, and has no magazine inside the weapon. The belt hangs outsdie, and has metal links that hold the cartridges together. A standard belt is either 50, 75 or 250 linked cartridges.