Check the sample and change your code accordingly
Label1.Text = "1";
if (Session["counter"] != null)
{
Label1.Text = Session["counter"].ToString();
Label1.Text = (Convert.ToInt16(Label1.Text) + 1).ToString();
}
if (!Page.IsPostBack)
{
Session["counter"] = Label1.Text;
}
Label1.Text = Label1.Text;