//CodeBehind:

    protected void Page_Load(object sender, EventArgs e)
    {
        //if (!IsPostBack)
        {
            for (int i = 1; i <= 5; i++)
            {
                ddl.Items.Add(new ListItem(i.ToString()));
            }
        }
    }

    protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
    {
        int nrUploads = 0;
        FileUpload fu;

        if(! int.TryParse(ddl.SelectedValue,out nrUploads))
        {
            return;
        }

        for(int i = 0; i < nrUploads; i++)
        {
            fu = new FileUpload();
            fu.Style.Add(HtmlTextWriterStyle.Display, "block");
            table.Rows[0].Cells[0].Controls.Add(fu);
        }
        TextBox tb = new TextBox();
        tb.TextMode = TextBoxMode.MultiLine;
        tb.Width = 200;
        tb.Height = 100;

        table.Rows[0].Cells[1].Controls.Add(tb);    
    }

//aspx file:

<%@ Page MasterPageFile="~/MasterPage.master"  Language="C#" AutoEventWireup="true" CodeFile="Upload.aspx.cs" Inherits="Upload" %>

<asp:Content runat="server" ContentPlaceHolderID="ContentPlaceHolder1">

<asp:DropDownList runat="server" ID="ddl" OnSelectedIndexChanged="ddl_SelectedIndexChanged"></asp:DropDownList>

<asp:ScriptManager ID="ScriptManager1" runat="server" />

<asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelId="UpdatePanel1" >
    <ProgressTemplate>
    <img src="App_LocalResources/loading.gif" alt="" />
    </ProgressTemplate>
</asp:UpdateProgress>

<asp:UpdatePanel runat="server" ID="UpdatePanel1">
<Triggers>
    <asp:AsyncPostBackTrigger ControlID="ddl" EventName="SelectedIndexChanged" />
</Triggers>
<ContentTemplate>

    <asp:Label ID="label1" runat="server"></asp:Label>
    <asp:Table ID="table" runat="server" BorderWidth="1">
        <asp:TableRow>
            <asp:TableCell>
            </asp:TableCell>
            <asp:TableCell>
            </asp:TableCell>
        </asp:TableRow>
    </asp:Table>
</ContentTemplate>

</asp:UpdatePanel>
</asp:Content>