Change a variable to each time that someone connects on the server.

Scripting
1 
using UnityEngine;
using System.Collections;

public class NetworkManager : MonoBehaviour {

    public bool isVisible;
    public GameObject chatSystem;
    string registeredGameName = "PhoenixGames_networking_test_server_multi-CUBEMOBA";
    bool isRefreshing = false;
    bool joined;
    float refreshRequestLength = 3.0f;
    HostData[] hostData;
    public GameObject characterSelectSystem;
    public GameObject characterSelectSystemGame;
    public string serverName;

    void Start(){
        isVisible = true;
    }

    private void StartServer(){
        Network.InitializeServer (16, 25002, false);
        MasterServer.RegisterHost (registeredGameName, serverName, "Server");
    }

    private void SpawnPlayer(){
        Debug.Log("Spawning Player...");
        characterSelectSystemGame = (GameObject)Network.Instantiate (characterSelectSystem, new Vector3(0f, 1f, 0f), Quaternion.identity,0);
        Network.Instantiate (chatSystem, new Vector3 (0, 0, 0), Quaternion.identity, 0);
    }

    void OnServerInitialized(){
        Debug.Log("Server Has Been Initialized");
        SpawnPlayer ();
    }

    void OnApplicationQuit(){
        if (Network.isServer) {
            Network.Disconnect(200);
            MasterServer.UnregisterHost();
        }
        if (Network.isClient) {
            Network.Disconnect(200);
        }
    }

    void OnPlayerDisconnect(NetworkPlayer player){
        Debug.Log ("Player disconnected");
        Network.RemoveRPCs (player);
        Network.DestroyPlayerObjects (player);
    }

    void OnMasterServerEvent(MasterServerEvent masterServerEvent){
        if (masterServerEvent == MasterServerEvent.RegistrationSucceeded) {
            Debug.Log("Registration Successful");
        }
    }

    public IEnumerator RefreshList(){
        Debug.Log ("Refreshing...");
        MasterServer.RequestHostList(registeredGameName);
        float timeStarted = Time.time;
        float timeEnd = Time.time + refreshRequestLength;
        while(Time.time < timeEnd){
            hostData = MasterServer.PollHostList ();
            yield return new WaitForEndOfFrame();
        }
        if (hostData == null || hostData.Length == 0) {
            Debug.Log ("There are no active servers");
        } else {
            Debug.Log(hostData.Length + " have been found");
        }
    }

    public void OnGUI(){
        if (isVisible) {
            serverName = GUI.TextField (new Rect (250f, 35f, 200, 20), serverName, 25);
        }
        if (Network.isServer) {
            GUILayout.Label ("Runnning a Server");
        } else if (Network.isClient) {
            GUILayout.Label ("Running as a Client");
        }

        if (Network.isClient) {
            if(!joined) {
                if (GUI.Button (new Rect (25f, 25f, 150f, 30f), "Join") && characterSelectSystem == null){
                    SpawnPlayer();
                    joined = true;
                }
            }
        }

        if (Network.isClient || Network.isServer) {
            return;
        }
        if (GUI.Button (new Rect (25f, 25f, 175f, 30f), "Host A Server")) {
            StartServer();
            isVisible = false;
        }
        if (GUI.Button (new Rect (25f, 65f, 175f, 30f), "Refresh List")) {
            StartCoroutine("RefreshList");
            isVisible = false;
        }
        if (hostData != null) {
            for (int i = 0; i < hostData.Length; i++){
                if (GUI.Button (new Rect (Screen.width/2, 65f+ (30f * i), 300f, 30f),hostData[i].gameName) && characterSelectSystem != null){
                    Network.Connect(hostData[i]);
                    characterSelectSystemGame.GetComponent<CharacterSelect>().myNumb += 1;
                }
            }
        }
    }
}

This is my code can someone help me! I want to change a variable to the character selection system that will be increased everytime someone connects! Thanks for your time!

2

Use a list on the server and add a NetworkConnection to that list maintained only by the server. Then, you just need to use the length of said list to change your variables.

Use [ServerCallback] or [Server] to limit the execution of the list only to the server.