In Mn2(CO)10, the number of CO molecules in between the metal atoms is 0. Let's break down the molecular structure and explain why.

Molecular Structure:
- The compound Mn2(CO)10 consists of two manganese (Mn) atoms and ten carbon monoxide (CO) molecules.
- The Mn atoms are bonded together, forming a dimeric structure.
- The CO molecules are coordinated to the Mn atoms, surrounding them.

Coordination:
- Each carbon monoxide (CO) molecule consists of a carbon atom (C) bonded to an oxygen atom (O).
- The oxygen atom has two lone pairs of electrons, making it a potential donor for coordination.
- The carbon monoxide (CO) molecules coordinate to the Mn atoms through the oxygen atom, forming a bond between the oxygen and the metal.
- The coordination number of Mn in this compound is 10, meaning that each Mn atom is coordinated to 10 CO molecules.

Arrangement:
- The CO molecules coordinate to the Mn atoms in a way that maximizes the coordination number.
- In Mn2(CO)10, the CO molecules are arranged in such a manner that they surround the Mn atoms, forming a coordination sphere.
- The CO molecules are not directly bonded to each other or forming a chain between the metal atoms.

Conclusion:
- Based on the molecular structure and coordination of Mn2(CO)10, there are no CO molecules in between the metal atoms.
- The CO molecules coordinate to the Mn atoms and surround them, but they do not bond to each other or form a chain between the metal atoms.
- Therefore, the correct answer is 0 CO molecules in between the metal atoms.