Hi anirudhp,

Will you please let me know for multiple selection which plugin you are using ?

Please share your code.

<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>

here the code i used

Hi anirudhp,

Refer the below code.

HTML

<asp:ListBox ID="ddlCOMMISION" SelectionMode="Multiple" CausesValidation="True" Height="50px"
    runat="server">
    <asp:ListItem>% ON BASIC RATE</asp:ListItem>
    <asp:ListItem>% ON NETT ORDER AMT</asp:ListItem>
    <asp:ListItem>Rs PER KG</asp:ListItem>
</asp:ListBox>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    $(function () {
        $('[id*=ddlCOMMISION]').on('change', function () {
            var selectedCount = 0;
            $('[id*=ddlCOMMISION] option').each(function () {
                if ($(this).is(':selected')) {
                    selectedCount++;
                }
                if (selectedCount > 2) {
                    alert("you can select max 2 of items");
                    $(this).removeAttr("selected");
                }
            });
        });
    });
</script>

Screenshot