动态增加一行Textbox框

//页面代码：

 <tr valign="middle">
                <td colspan="12" align="left"><asp:TextBox ID="txtUNAccessories3" TextMode="MultiLine" Columns="95" runat="server"
                       ></asp:TextBox><input type="button"  name="addbtn"  value="增加一条"
                        onclick="FileNumChange(true);" id="Button3" runat="server"/> </td>
            </tr>
          
          
         
                </table>
             
                <asp:Panel  ID="Panel1" runat="server"  Width="100%">
                   <asp:TextBox  style="display:none" ID="txtUNAccessories5" TextMode="MultiLine" Columns="95" runat="server"
                       ></asp:TextBox>
         
                    </asp:Panel>
                   
                    <asp:Panel ID="Panel2" runat="server"  Width="100%" >
                    <asp:TextBox  style="display:none" ID="txtUNAccessories6" TextMode="MultiLine" Columns="95" runat="server"
                       ></asp:TextBox>
                    </asp:Panel>
                   
                    <asp:Panel ID="Panel3" runat="server"  Width="100%" >
                     <asp:TextBox  style="display:none" ID="txtUNAccessories7" TextMode="MultiLine" Columns="95" runat="server"
                       ></asp:TextBox>
                    </asp:Panel>
                   
                    <asp:Panel ID="Panel4" runat="server"  Width="100%" >
                    <asp:TextBox style="display:none" ID="txtUNAccessories8" TextMode="MultiLine" Columns="95" runat="server"
                       ></asp:TextBox>
                    </asp:Panel>
                   
                    <asp:Panel ID="Panel5" runat="server"  Width="100%" >
                    <asp:TextBox  style="display:none" ID="txtUNAccessories4" TextMode="MultiLine" Columns="95" runat="server"
                       ></asp:TextBox>
                    </asp:Panel>
             
                   
                </td>
            </tr>

//要调用的脚本

 var defaultNum2 = 0;
function FileNumChange2(add2)
{
 var arr2 = new Array();
 arr2[0] = document.getElementById("<%=txtAdvice5.ClientID%>");
 arr2[1] = document.getElementById("<%=txtAdvice6.ClientID%>");
 arr2[2] = document.getElementById("<%=txtAdvice7.ClientID%>");
 arr2[3] = document.getElementById("<%=txtAdvice8.ClientID%>");
 arr2[4] = document.getElementById("<%=txtAdvice4.ClientID%>");
 if (add2 && defaultNum2<6){
  defaultNum2++;
 }else if (!add2 && defaultNum2>1){
  defaultNum2--;
 }
 
 for(var i=0; i<arr2.length; i++){
  if (i >= defaultNum2){
   arr2[i].disabled = true;
   arr2[i].style.display = "none";
  }else{
   arr2[i].disabled = false;
   arr2[i].style.display = "block";
  }
  
 }
 if(defaultNum2>5)
  {
  alert('由于数据库限制，此处只能填写8条，超出者请以附件提交！！！');
  }
}