<form name="frm" method="post" action="?">
<p><input type="checkbox" name="title[]"></p>
<p><input type="checkbox" name="title[]"></p>
<p><input type="checkbox" name="title[]"></p>
<p><input type="checkbox" name="title[]"></p>
<p><input type="checkbox" name="title[]"></p>
<input type="button" value="Alert" onclick="doclick()" />
 </form>
<script type="text/javascript">
 function doclick() {
  var checked = 0;
  var titles =  frm.elements['title[]'];
  for (var i = 0; i < titles.length; i++) {
     if (titles[i].checked) {
        checked++;
     }
  }
  alert(checked);
}
</script>