Optimal Polygon Count for a Plane for iOS

I have several large planes with textures mapped as foreground, mid-ground, and background display in a 2.5D iOS app. I assume I could make more optimal planes in a 3D program than the default Unity primitives. How many polygons should I use to create the planes? One big polygon or some low number of polys? Would it help if I made them all one mesh?

I do not know the exactly polygon count of the built in plane(something under 200 polys) it is not that huge, but when you use a 3D program and create a simple plane it will only have 2 polygons.

We are presently doing some RnD fora game on the ipad 1.
We were able to make a scene with 150 000 triangles together with 30 2048 textures and it all runs smoothly ( 30 fps).
Just avoid big planes with no subdivisions, try having an equal amount of triangles for efficient culling.