Emtpy stack ASP

I want to create dinamically some button in a form in ASP.

The aspx code is this:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Scadenze.aspx.cs" Inherits="Scadenze" MasterPageFile="~/Site.Master" %>

<asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" Runat="Server">

<asp:Label runat="server" ID="Label1" ></asp:Label>
<form id="form1" runat="server">
                    <asp:Button ID="Button1" runat="server" Text="Add" onclick="Button1_Click" />

The code of Button1_Click is this

protected void Button1_Click(object sender, EventArgs e) { for (int i = 0; i < 4; i++) //specifying the condition in for loop for creating no. of textboxes {

    TextBox tb = new TextBox();   //Create the object of TexBox Class
    tb.ID = i.ToString();                // assign the loop value to textbox object for dynamically adding Textbox ID
    Form.Controls.Add(tb);        // adding the controls



When I run the application it give me an error Exceptions details: Empty stack "Additional information: A page can contain only one tag Form server site."

How can I solve it? Thank you very much!