<form id="form1" runat="server">
       <asp:Panel runat="server" ID="pnl"/>
       <asp:Button ID="btn1" runat="server" OnClick="Button1Click" Text="проверить" />
       <asp:Button ID="btn2" runat="server" OnClick="Button2Click" Text="Добавить контрол" />
    </form>

        protected void Button1Click(object sender, EventArgs e)
        {
            var i = pnl.Controls.Count;
        }

        protected void Button2Click(object sender, EventArgs e)
        {
            var s = new Label {ID="lblNew", Text = "текст"};
            pnl.Controls.Add(s);
            pnl.DataBind();
        }