Set the max number of items that can be selectable in ListBox in asp.net and c# using java scripts

<script type="text/javascript" language="javascript">
    function onClientSelectedIndexChangingHandler(source, arguments) {
        var listbox = document.getElementById('<%=ddlCOMMISION.ClientID%>');
        var selectedCount = 0;
        for (var index = 0; index < listbox.options.length; index++) {
            if (listbox.options[index].selected)
                selectedCount += 1;
        }
        if (selectedCount < 4)
            arguments.IsValid = true;
        else
            arguments.IsValid = false; 
    }
</script>
 
<asp:ListBox ID="ddlCOMMISION"  SelectionMode="Multiple" CausesValidation="True" Height="50px" runat="server">
    <asp:ListItem>--Select--</asp:ListItem>
    <asp:ListItem>% ON BASIC RATE</asp:ListItem>
    <asp:ListItem>% ON NETT ORDER AMT</asp:ListItem>
    <asp:ListItem>Rs PER KG</asp:ListItem>
</asp:ListBox>
<asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="Maximum Countries you can select are 5"
    ClientValidationFunction="onClientSelectedIndexChangingHandler"></asp:CustomValidator>