In [Co2(CO)8], the number of CO molecules lying between the metal atoms is 2. Let's break down the structure of the compound and understand why this is the correct answer.

Structure of [Co2(CO)8]:
- The compound consists of two cobalt (Co) atoms bonded together.
- Each cobalt atom is surrounded by eight carbon monoxide (CO) molecules, making a total of 16 CO molecules in the compound.

Determining the number of CO molecules between the metal atoms:
- To find the number of CO molecules between the metal atoms, we need to identify the bridging and terminal CO ligands.
- Bridging ligands are the ones that connect two metal atoms, while terminal ligands are attached to only one metal atom.

Identification of bridging and terminal CO ligands:
- In [Co2(CO)8], the CO molecules can be divided into two groups:
1. Bridging CO ligands: These ligands connect the two cobalt atoms.
2. Terminal CO ligands: These ligands are attached to only one cobalt atom.

Calculating the number of bridging CO ligands:
- Each bridging CO ligand connects two metal atoms, so it contributes to the number of CO molecules lying between the metal atoms.
- In [Co2(CO)8], there are a total of 16 CO molecules.
- Each cobalt atom is surrounded by eight CO molecules, but only four of them are terminal ligands.
- Hence, the remaining 12 CO molecules are bridging ligands, connecting the two cobalt atoms.

Therefore, the number of CO molecules lying between the metal atoms in [Co2(CO)8] is 12.

To summarize:
- The compound [Co2(CO)8] consists of two cobalt atoms bonded together.
- There are a total of 16 CO molecules in the compound.
- Out of these, 12 CO molecules act as bridging ligands, connecting the metal atoms.
- The remaining four CO molecules are terminal ligands, attached to only one metal atom.

Hence, the correct answer is 2, which represents the number of CO molecules lying between the metal atoms.