Add vertical Scroll to Bootstrap jQuery MultiSelect Plugin in ASP.Net

Please refer below code,you can adjust maxHeight as per your requirement(i.e. Number of items to be shown).

HTML

<div>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
    <link href="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css"
        rel="stylesheet" type="text/css" />
    <script type="text/javascript" src="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js"></script>
    <link href="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/css/bootstrap-multiselect.css"
        rel="stylesheet" type="text/css" />
    <script src="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/js/bootstrap-multiselect.js"
        type="text/javascript"></script>
    <script type="text/javascript">
        $(function () {
            $('[id*=lstFruits]').multiselect({
                includeSelectAllOption: true,
                maxHeight: 200
            });
        });
    </script>
    <asp:ListBox ID="lstFruits" runat="server" SelectionMode="Multiple">
        <asp:ListItem Text="Mango" Value="1" />
        <asp:ListItem Text="Apple" Value="2" />
        <asp:ListItem Text="Banana" Value="3" />
        <asp:ListItem Text="Guava" Value="4" />
        <asp:ListItem Text="Orange" Value="5" />
        <asp:ListItem Text="Mango" Value="1" />
        <asp:ListItem Text="Apple" Value="2" />
        <asp:ListItem Text="Banana" Value="3" />
        <asp:ListItem Text="Guava" Value="4" />
        <asp:ListItem Text="Mango" Value="1" />
        <asp:ListItem Text="Apple" Value="2" />
        <asp:ListItem Text="Banana" Value="3" />
        <asp:ListItem Text="Guava" Value="4" />
        <asp:ListItem Text="Orange" Value="5" />
        <asp:ListItem Text="Orange" Value="5" />
    </asp:ListBox>
    <asp:Button Text="Submit" runat="server" OnClick="Submit" />
</div>

 Code

protected void Submit(object sender, EventArgs e)
{
    string message = "";
    foreach (ListItem item in lstFruits.Items)
    {
        if (item.Selected)
        {
            message += item.Text + " " + item.Value + "\\n";
        }
    }
    ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", "alert('" + message + "');", true);
}

Screenshot