How to Limit Number Of Characters in TexBox Inside DataGrid

I have One One DataGrid , in this DataGrid i have one More Child DataGrid.

In Child DataGrid I have One ItemTemplate...In this ItemTemplate i have one TextBox.

I have to Limit the Number Of Characters in this TextBox, When user eneter into textbox.

onkeypress="return this.value.length<=20 (This is working )

 <asp:TextBox ID="txtfeedback" Style="padding-right:10px" TextMode="MultiLine" Columns="38" Rows="2" runat="server"  onkeypress="return this.value.length<=20" ></asp:TextBox>

When i enter characters , it allowing 20 characters and after that it not allowing eneter characters.

But i need to show in some Lable like if you type 12 characters in textox we need to show 8 characters remaining.

like that i want output, but the TextBox is inside GridView.

Please hellp me,

Thanks.