Hi there:
I built a few simple 3d models in 3D max, after exported FBX and dragged into Unity, the polygon number increased dramaticly. Can anyone tell me how to keep the same polygon number?
You can’t…the numbers in Max are not “real”, since any quads need to be triangulated, and vertices need to be split because of things like UV seams and normals. For example, a cube with correct lighting can’t be 8 vertices, it must be 24 (4 for each side, since normals are per-vertex).
–Eric
As a side note, this counts even for most 3D formats like FBX. Every vertice of a face gets written here, the vertices gets split. A cube has 24 vertices in a FBX file. Interestingly Obj doesnt split the vertices that way, but references the corresponding vertices by its index. Here a cube just has 8 written vertices.Thats one of the reasons why a Obj file is much smaller compared to the same file written as FBX.
As a consequence you can save some space in your project folder here when you use Obj over FBX. Unfortunately Obj cannot deal with animation. Its just good for static stuff. And unfortunately the Obj importer is broken since years. It´s just able to deal proper with one material per file. Its a known bug, but Unity doesn`t mind about it.
Unity itself splits the vertices then again though, and triangulates the mesh internal. In the exe you won`t find FBX or Obj. And then we are back at what Eric said.
Actually no, it depends on what the file exporter does. For example, if you make a cube in Blender and export it as FBX, even if you set the faces as solid rather than smooth, you get 8 vertices in the file. You have to either manually split the vertices yourself, or else mark all edges as sharp and use the edge split modifier, in order to get 24 vertices in the file. It’s usually not worth trying to save space by messing with model formats anyway, since the space used by model files will probably be totally dwarfed by texture and sound files, and disk space is cheap. Use what’s most convenient, which in the case of Blender is usually .blend files, so you don’t need to spend time exporting anything manually.
–Eric
True. You don`t really save much space in your project file. The biggest space eater is Unity itself with all its meta data.
I count 24 
Maybe we misunderstand each other here. I don`t talk about shading, just about the number of vertices that gets stored in the file.
You’re counting 24 floats…it takes 3 floats to make a vertex. Unless something’s changed recently, we’re still modeling things in 3 dimensions. 
–Eric
Oops, you are right 