If you add a MeshInstance3D with a BoxMesh and click on the toolbar “Mesh” icon and “View UV1” you can see the 3x2 UV layout.
That is how the BoxMesh applies the material texture to its triangle faces.
So what you do is design a texture that way so that the texture colors map to the faces that you want.
You only use one texture for all faces, not multiple textures for each face.
If you need to use a different texture for each single face you need to write a custom shader that allows for multiple texture uniforms or change the entire mesh surfaces procedual, e.g. adding a new surface for each single face with either the SurfaceTool or the ArrayMesh API.