how the dynamic textboxes appear on postback in my code please help

protected void Button1_Click(object sender, EventArgs e)
    {
      if (txtQuantity.Text != "")
        {
            if (Page.IsPostBack)
            {
                for (int i = 0; i <= Convert.ToInt16(txtQuantity.Text) / 3; i++)
                {


                    Label la = new Label();
                    la.Text = ddlProductName.SelectedItem.Value + (i + 1).ToString();
                    la.ID = "abc1" + i;

                    PlaceHolder1.Controls.Add(la);

                    //la = FindControl(String.Format(i)) as Label;




                    TextBox txtbox = new TextBox();
                    // txtbox.Text = "Textbox - " + i.ToString();


                    PlaceHolder1.Controls.Add(new LiteralControl("<input id='txt' name='Textbox" + i + "'type='text'  />"));

                    PlaceHolder1.Controls.Add(new LiteralControl("<br />"));
                }
                for (int j = (Convert.ToInt16(txtQuantity.Text) / 3) + 1; j <= Convert.ToInt16(txtQuantity.Text) * 2 / 3; j++)
                {


                    Label la1 = new Label();
                    la1.Text = ddlProductName.SelectedItem.Value + (j + 1).ToString();
                    la1.ID = "abc2" + j;

                    PlaceHolder2.Controls.Add(la1);

                    //la = FindControl(String.Format(i)) as Label;




                    TextBox txtbox1 = new TextBox();
                    // txtbox.Text = "Textbox - " + i.ToString();
                    PlaceHolder2.Controls.Add(new LiteralControl("<input id='txt' name='Textbox" + j + "'type='text'  />"));
                    PlaceHolder2.Controls.Add(new LiteralControl("<br />"));
                }
                for (int k = (Convert.ToInt16(txtQuantity.Text) * 2 / 3) + 1; k < Convert.ToInt16(txtQuantity.Text); k++)
                {


                    Label la2 = new Label();
                    la2.Text = ddlProductName.SelectedItem.Value + (k + 1).ToString();
                    la2.ID = "abc3" + k;

                    PlaceHolder3.Controls.Add(la2);

                    //la = FindControl(String.Format(i)) as Label;




                    TextBox txtbox1 = new TextBox();
                    // txtbox.Text = "Textbox - " + i.ToString();


                    PlaceHolder3.Controls.Add(new LiteralControl("<input id='txt' name='Textbox" + k + "'type='text'  />"));

                    PlaceHolder3.Controls.Add(new LiteralControl("<br />"));
                }
            }

        }
    }