It should only have 8
It should have 24, because each face needs 4 separate vertices for the normals and UVs. The faces can’t share normals and UVs (well, technically they could, but the lighting and texture would look completely screwed up).
Each side is made of two triangle, but these two triangles share 2 vertices. This gives 4 vertices per side. In total 4*6=24 vertices.
The reason why the vertices cannot be shared, is that they have different normals.