JavaScript

function Count(text, long) {

            var maxlength = new Number(long);

            var remChar = maxlength - document.getElementById('<%=txtifsccode.ClientID%>').value.length;

            if (remChar > -1) {

                document.getElementById('<%=span_count.ClientID%>').innerHTML = remChar;

             }

             if (document.getElementById('<%=txtifsccode.ClientID%>').value.length > maxlength) {

                text.value = text.value.substring(0, maxlength);

            }

        }

<asp:TextBox ID="txtifsccode" runat="server" onblur="return AllowIFSC();" onKeyUp="Count(this,11);" MaxLength="11" Width="200" Style='text-transform: uppercase'></asp:TextBox>

                                    <span id="span_count" style="color: red;" runat="server"></span>