How to do UV mapping on precodurally generated mesh

Hi, I know it seems like a basic question but there is some point I dont understand. For a cube normally it is possible to build a cube with 8 vertices. But I think that raises a problem when you want to have a proper shading and uv mapping. Because that way you will not be able to show the same texture on each face. So do I have to assign 4 vertices for each face? A completely unrelated question: Does the vertex count have burden on speed graphic card or just consumes memory? Because when you think of it triangle count affects speed but vertices are not displayed unless referenced by a triangle. It just sits on memory. I want to figure these out because I am working on a large geometry project and don't want to waste any performance.