why is Unity got more vertices than 3dsmax

3DS Max will single-counts shared vertices with separate normals, but when importing, Unity will split per-vertex normals. If your mesh does not smooth the normals across vertices (smoothing groups), you will get an increase in vertices because each face will have each of its vertices with separate normals generate a separate vertex with a separate normal in the export rather than sharing the vertices with other faces. To resolve this, you should apply smoothing groups to your model, sharing smoothing groups across all smooth faces.

For example:

A plane with a pivot positioned at the lower-left corner can be defined as:

Vector3[] vertices = new Vector3[6];
Vector3[] normals = new Vector3[6];
Vector2[] uv = new Vector2[6];
int[] triangles = new int[6];
uv[0] = new Vector2(0.0f,0.0f);
uv[1] = new Vector2(0.0f,1.0f);
uv[2] = new Vector2(1.0f,1.0f);
uv[3] = new Vector2(0.0f,0.0f);
uv[4] = new Vector2(1.0f,1.0f;
uv[5] = new Vector2(1.0f,0.0f);
for(int i = 0; i < 6; i++) {
    vertices[i] = new Vector3(uv[i]);
    normals[i] = Vector3.forward;
    triangles[i] = i;
}

or:

Vector3[] vertices = new Vector3[4];
Vector3[] normals = new Vector3[4];
Vector2[] uv = new Vector2[4];
int[] triangles = new int[6];
uv[0] = new Vector2(0.0f,0.0f);
uv[1] = new Vector2(0.0f,1.0f);
uv[2] = new Vector2(1.0f,1.0f);
uv[3] = new Vector2(1.0f,0.0f);
for(int i = 0; i < 4; i++) {
    vertices[i] = new Vector3(uv[i]);
    normals[i] = Vector3.forward;
}
for(int i = 0; i < 3; i++) triangles[i] = i;
triangles[3] = 0; //shared
triangles[4] = 2; //shared
triangles[5] = 3;

Notice how the exact same plane can have nearly double the vertices. Because the normals were not shared in 3DS max across the triangle faces, each face gets each of its vertices separated as in the first example. The same plane with a smoothing group across the two triangles or defined as the same face in an editable poly (essentially hiding the edge and smoothing the normals) will generate the second example.