Is it possible to make this model here have only one face on it?
No -> a face is also known as a polygon, and a polygon can be comprised of a maximum of 4 faces. Thus, no,
That would be an n-gon. The current version of blender only supports 3 or 4 sided faces. There is development to use n-gons. Look up bmesh builds at graphicall.org
As you have posted in the game engine forum I assume this must be for a game. The game engine converts everything to triangles anyway so there would be little advantage.
You could try “faking” by using a texture with a circle(in your case), and put(apply) it on a plane.
Still, you could use “Convert triangles to quads” (by pressing Alt + J simultaneously, and Ctrl + T for the inverse effect)
Richard’s correct - the BGE converts everything to triangles, so N-Gons wouldn’t have much use for the game engine. For curiosity’s sake, why would you want to make that shape with a single polygon?
@Richard Marklew yes it is for a game it would help to make my models lower poly but ok thanks for the help