1. Identify the complex

   The given compound is: $CoCl_3\cdot 4NH_3$

   Since the question mentions the trans-complex, this corresponds to the coordination compound: $trans-[Co(NH_3)_4Cl_2]Cl$

   Here:

   • Inside the coordination sphere: $4NH_3$ and $2Cl^-$
   • Outside the coordination sphere: $1Cl^-$

2. Find the neutral ligands in the coordination sphere

   The neutral ligands present inside the coordination sphere are ammonia molecules: $4NH_3$

3. Understand the replacing ligand

   Ethylenediamine (en) is a bidentate neutral ligand, so:

   • 1 molecule of en occupies 2 coordination sites

4. Replace all neutral ligands

   We need to replace all $4$ ammonia ligands by en.

   Since: $1\,en \equiv 2\,NH_3$

   Therefore, the number of en molecules required is: $\frac{4}{2} = 2$

5. Nearest integer

   The value is already an integer: $2$

Final Answer

$\boxed{2}$