checkbox based input item

If you want to submit form or some data, use <input type="checkbox" onchange="myfunction(this)" ...>

Then write JS function myfunction (you can name it whatever makes sense to you, of course), and make that function post data to server. If you use jQuery it'll be $.post() function.

First row only affected. remaining row no output

 echo '<td  align="center"><input type="number" min="0" max="999" name="cnt" value="0"   onkeyup="sum();" id="texttwo" disabled></td>';

echo '<td align="center"><input type="checkbox" value="' . $row['certificate_id'] . '" name="certid[]" id="chb"></td>';

<script src="//ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script>
<script type="text/javascript">
  document.getElementById('chb').onchange = function() {
    document.getElementById('texttwo').disabled = !this.checked;
};
</script>