Mesh.subMeshCount  

function Start() {
var mesh : Mesh = GetComponent(MeshFilter).mesh;
Debug.Log("Submeshes: " + mesh.subMeshCount);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Start() {
Mesh mesh = GetComponent<MeshFilter>().mesh;
Debug.Log("Submeshes: " + mesh.subMeshCount);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Start():
mesh as Mesh = GetComponent[of MeshFilter]().mesh
Debug.Log(('Submeshes: ' + mesh.subMeshCount))