im sorry , i didnt get you the first time.

use client event "onkeypress"

<asp:TextBox ID="TextBox1" runat="server" onkeypress="rui()">
        </asp:TextBox>

and create a javascript function

<script language="javascript" type="text/javascript">
        function rui() {
            alert('max 20');
        }
    </script>

everytime the key is pressed it will check the function.