How can i return List & Integer from C# to Javascript

You can use JavaScriptSerializer:

var myClass = new ClassName();

... 

var jSon = new JavaScriptSerializer();
var OutPut = jSon.Serialize(myClass);

Response.Write(OutPut);

You have to import this namespace: System.Web.Script.Serialization

UPDATE:

You can use jQuery to post the request:

<script type="text/javascript">
    $(document).ready(function() {
        $.ajax({
            type: 'POST',
            url: 'WebForm2.aspx',
            data: {},
            dataType: 'json',
            complete: function(XMLHttpRequest, textStatus) {
                var Response = $.parseJSON(XMLHttpRequest.responseText);
                alert(Response.Classes[0].Name);
            }
        });
    });
</script>

and this is the code-behind:

public partial class WebForm2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

        var classWrapper = new ClassWrapper();

        classWrapper.Classes.Add(new ClassName() { Name = "Test 1" });
        classWrapper.Classes.Add(new ClassName() { Name = "Test 2" });
        classWrapper.Classes.Add(new ClassName() { Name = "Test 3" });
        classWrapper.Count = classWrapper.Classes.Count;


        var jSon = new JavaScriptSerializer();
        var OutPut = jSon.Serialize(classWrapper);

        Response.Write(OutPut);
    }
}

public class ClassWrapper
{
    public ClassWrapper()
    {
        Classes = new List<ClassName>();
    }
    public List<ClassName> Classes { get; set; }
    public int Count { get; set; }
}

public class ClassName
{
    public string Name { get; set; }
}

Few tricks for ASP.NET (in MVC would be easier).

in the HTML of WebForm2.aspx remove all the HTML but leave page directive:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm2.aspx.cs" Inherits="AutocompleteASPNET.WebForm2" %>