I try th combine 15000 cube Meshes into one single Mesh.

I do it like this:

// i have a Dictionary of cubes which i reference to some information, 
// and contains 15000 cubes
public Dictionary<string, GameObject> cubes;
// i also have an Mesh created in the Start() function
public Mesh mesh = new Mesh();

CombineInstance[] combineMeshes = new CombineInstance[cubes.Values.Count];
int count = 0;
foreach (GameObject c in cubes.Values)
{
    MeshFilter mF = c.GetComponent<MeshFilter>();
    combineFileMeshes[count].mesh = mF.mesh;
    combineFileMeshes[count].transform = c.transform.localToWorldMatrix;
    count++;
}

mesh = Mesh.CombineMeshes(combineMeshes);
mesh.Optimize();

After i did this, the cubes existed, and the cubes in the mesh should look the same. But they dont. Are there limitations how many objects you can combine in a single Mesh?