I added a single flat quad into the default Unity SampleScene (with the skybox removed so it’s basically empty other than the standard camera and light) to see how many polygons or tris a quad was (I assume tris is what Unity calls polygons) and I thought the number would be 2, but it increased the tris number to 10 more and the verts number to 20 more.
Can someone explain to me what these numbers actually mean, because obviously it’s not what I thought?
Shadows and light passes are counted seperately. Also any post processing effects will also have an effect, So it highly depends on your lighting model and rendering path as well as on your shadow settings. (shadow receiver / caster)
The actual triangles and vertices that a mesh has can be seen in the inspector when you inspect the mesh itself. If you have a MeshFilter component just double click the mesh field to inspect the actual mesh. At the bottom in the preview area it tells you how many vertices, triangles and what other vertex attributes the mesh has.
The stats window shows how many vertices and triangles are actually drawn. A shader with multiple passes will render the object multiple times.
The statistics window shows stats for everything rendered to produce the frame. Lights might require extra passes or shadowmaps. You can see these by opening the Frame Debugger window. You can click each draw call to see how many verts it renders. All draw calls are added up and the total is shown in the statistics window. You can have the Frame Debugger open and disable/enable lights and shadows to see how the draw calls change.
To see how many verts a mesh has, you can select it by double clicking it in the Mesh Filter or selecting it in the project window. Then you can expand the bottom of the inspector to see a preview and the number of verts and tris.