Edit in JSFiddle

var $inputs = $("ul>li>input");
var max = -1;
$inputs.each(function () {
    max = $(this).val() > max ? $(this).val() : max
});
var $list = $("ul:first");
$list.append('<li><input type="hidden" value="' + (parseInt(max, 10) + 1) + '" name="order"></li>');
alert($list.html());

<ul>
 <li><input type="hidden" value="7" name="order"></li>
 <li><input type="hidden" value="3" name="order"></li>
 <li><input type="hidden" value="6" name="order"></li>
 <li><input type="hidden" value="5" name="order"></li>
 <li><input type="hidden" value="1" name="order"></li>
 <li><input type="hidden" value="2" name="order"></li>
</ul>