How to count every checked checkboxes

you can do it this way

$(document).ready(function(){
$('input[type="checkbox"]').click(function(){
    alert($('.test:checked').length);

});
});

HTML i used

<input type="checkbox" name="test" class="test" value=""/>  
<input type="checkbox" name="test" class="test" value=""/>  
<input type="checkbox" name="test" class="test" value=""/>  
<input type="checkbox" name="checkAll" class="checkAll" value=""/>

Hope this helps

You can use a function to update the counter :

function updateCounter() {
    var len = $("#general-content input[name='wpmm[]']:checked").length;
    if(len > 0){
       $("#general i .counter").text('('+len+')');
    } else { 
       $("#general i .counter").text(' ');
    }
}

and call this function when a checkbox's state is changed (including the selectAll checkboxes)

Here is an updated jsFiddle : http://jsfiddle.net/8PVDy/4/

$('input[type="checkbox"]:checked').length