Typically there will be a conceptual “mesh”, that is actually comprised of multiple “SubMeshes”. When you export a cube with a different material on each face, Unity will import that as a Mesh with one SubMesh for each face, and each SubMesh will only have a single material. 

lalamung said:

i really need this solution bro

What solution do you need exactly? 

You got a problem? Yo, I'll solve it.

I have no experience in Unity nor other dev software, I use Maya for 3D Modelling and Animation. I know to rigging too