try
<span id="charcount" style="color:red">0</span> characters entered.
<asp:TextBox ID="txtMessage" onkeyup="countChars();" onkeydown="countChars();"
onmouseout="countChars();" runat="server" Width="95%"
placeholder="Message........" TextMode="MultiLine" Height="220px" MaxLength="2"
BorderColor="Blue"></asp:TextBox>
function countChars() {
var textBoxClientID = '<%= txtMessage.ClientID%>'
var len = document.getElementById(textBoxClientID).value.length;
document.getElementById('charcount').innerHTML = len;
}