Problem numbering Clients

I'm trying to make a Networking game, and I want to assign each player an unique number from 0 to the player numbers-1. I made a code, but it doesn't working:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.Networking;
 using UnityEngine.UI;
 
 public class OptionSync : NetworkBehaviour {
 
     public int myClientNum = -1;
     [SyncVar]
     public int nextClientNum= 0;
     
     void Update () 
     {
         if (isClient && myClientNum >=0)
         {
             GetComponentInChildren<Text>().text ="index:"+myClientNum;
         }
     }
     
     public override void OnStartClient()
     {
         base.OnStartClient();
         myClientNum = nextClientNum;
         CmdCreated();
     }
 
     [Command]
     void CmdCreated()
     {
         nextClientNum++;
     }
 }
 

The idea is to make a synced variable with the last index and to make a variable for the actual index. In the update I'm using a text to display the number. At the start of the Client I'm overriding the OnStartClient to assign the index. After that I'm calling the command to increase the synced nextClientNum. I think it should work, but when I run 3 instances I get a Client with index:0 the others has index:1.