I think I might know what you're asking. You can add as many vertexes and trianlges to a surface as you want (or the graphics card can handle). Vertexes are basically points in 3D space and are not visible. They are basically placeholders for defining triangles. When you create the triangle, then the triangle will be visible, assuming the normals are pointing in the correct direction. The surface is basically another placeholder which isn't visible. It is something to put vertexes and triangles on.
Think of it this way, suppose you want to draw a picture on a piece of paper. You start with nothing, then you get yourself a surface, i.e. the piece of paper. Now the paper is blank without any images on it at all; however the paper does have properties which will affect the final image. It's color, texture, thickness, etc...
Now you need something to draw with, so you grab a pencil. In 3D you're "drawing implement" is the vertex. You draw with the pencil by dragging it on the paper, in 3D you draw by connecting the vertexes with lines. Once again, the pencil has it's own properties which affect the image. Color, width, hardness, percentage of carbon/lead, etc...
Now you actually draw the image on the paper by dragging the pencil on the surface. Once again, properties can be affected by pressure, direction, dullness of pencil, etc...
Now like I said before, in 3D, you "draw" by connecting the vertexes with lines to create polygons. Some rendering engines allow various types of polygons, square, rectangular, octagonal, etc... But for games, it is more efficient to draw the polygons, and more consistent accross hardware, if all the vertexes in the polygon lie on the same plane. The only way to assure this is to allow only three vertexes, i.e. a triangle. No matter how you draw the triangle, the three vertexes making it up will always be on the same plane. The creators of DirectX decided, with DirectX being made mainly for games, to use triangles exclusively for 3D rendering.
Now for your question, since the surface is not visible until you add the triangles, and since the AddTriangle command only accepts 3 vertexes, there is really no point in adding more, unless you want to make more than one triangle. Even if there was a way, it would be inconsistent among graphics cards.
Did I sufficiently confuse you yet?
Think of it this way, suppose you want to draw a picture on a piece of paper. You start with nothing, then you get yourself a surface, i.e. the piece of paper. Now the paper is blank without any images on it at all; however the paper does have properties which will affect the final image. It's color, texture, thickness, etc...
Now you need something to draw with, so you grab a pencil. In 3D you're "drawing implement" is the vertex. You draw with the pencil by dragging it on the paper, in 3D you draw by connecting the vertexes with lines. Once again, the pencil has it's own properties which affect the image. Color, width, hardness, percentage of carbon/lead, etc...
Now you actually draw the image on the paper by dragging the pencil on the surface. Once again, properties can be affected by pressure, direction, dullness of pencil, etc...
Now like I said before, in 3D, you "draw" by connecting the vertexes with lines to create polygons. Some rendering engines allow various types of polygons, square, rectangular, octagonal, etc... But for games, it is more efficient to draw the polygons, and more consistent accross hardware, if all the vertexes in the polygon lie on the same plane. The only way to assure this is to allow only three vertexes, i.e. a triangle. No matter how you draw the triangle, the three vertexes making it up will always be on the same plane. The creators of DirectX decided, with DirectX being made mainly for games, to use triangles exclusively for 3D rendering.
Now for your question, since the surface is not visible until you add the triangles, and since the AddTriangle command only accepts 3 vertexes, there is really no point in adding more, unless you want to make more than one triangle. Even if there was a way, it would be inconsistent among graphics cards.
Did I sufficiently confuse you yet?