<form id="frm_test">
<?php
     for($i = 0 ; $i < 10 ; $i++) {
          echo '<input type="checkbox" name="checkbox[]" onclick="countCheck(\'frm_test\')" />';
     }
?>
<span id="count">&nbsp;</span>
</form>
<script type="text/javascript">
function countCheck( frm ) {
     ch = 0;
     var form = document.getElementById(frm);
     var inps = form.getElementsByTagName('input');
     for(var i = 0 ; i < inps.length ; i++) {
          inp = inps[i];
          if ( inp.type == 'checkbox' && inp.checked ) {
               ch++;
          };
     };
     document.getElementById('count').innerHTML = ch;
}
</script>