Update a table when changing a select with data displayed by a while, php

My problem is that I have a table of products and the number of pieces chosen, but I have the option that the pieces are changed by a select and consequently the subtotal is changed, and the total of all the buy, but it only works with the first data shown by the while , but does not change it with the others

This is where I show the aggregated data and here I have the select preloaded with the number of pieces selected, and can also be modified from select

  <table class="table table-hover table-condensed table-bordered">
 <tr>
  <th>Producto</th>
  <th>Precio</th>
  <th>Piezas</th>
  <th>Subtotal</th>
  <th>Eliminar</th>
</tr>
<?php foreach ($p as $registros ): ?>
  <?php if ($registros[piezas]!=0 and  $registros[subtotal]!=0) {
    $datos=$registros[id];

    $total=$total + $registros['subtotal'];
    ?>
    <input type="hidden" name="" value="<?php echo $registros[id]; ?>" id="id_">
    <tr>
      <th><?php echo $registros[producto]; ?></th>
      <th><?php echo $registros[precio]; ?></th>
      <th><select class="" name="" id="piezas">
        <option value="<?php echo $registros[piezas]; ?>"><?php echo $registros[piezas]; ?></option>
        <?php if (  $registros[piezas]!="1" ): ?>
          <option value="1">1</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="2" ): ?>
          <option value="2">2</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="3" ): ?>
          <option value="3">3</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="4" ): ?>
          <option value="4">4</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="5" ): ?>
          <option value="5">5</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="6" ): ?>
          <option value="6">6</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="7" ): ?>
          <option value="7">7</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="8" ): ?>
          <option value="8">8</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="9" ): ?>
          <option value="9">9</option>
        <?php endif; ?>
        <?php if (  $registros[piezas]!="10" ): ?>
          <option value="10">10</option>
        <?php endif; ?>
      </select> </th>
      <th><?php echo $registros[subtotal]; ?></th>
      <th> <button class="btn btn-danger "  value="<?php echo $datos ?>"onclick="preguntarpedido(<?php echo $datos; ?>)"><i class="fas fa-minus-circle"></i></button></th>
    </tr>

    <?php
  } ?>
<?php endforeach; ?>
<tr>
  <td colspan="6">TOTAL: <?php
  echo $total;
?></td>
</tr>

this function the charge until the end, and is responsible for changing the pieces

<script type="text/javascript">
$(document).ready(function() {
    $('#piezas').change(function(){
      $.ajax({
          type:"post",
          data:'piezas=' + $('#piezas').val()+
          '&id=' + $('#id_').val(),
          url:'../php/pedidoscambiar.php',
          success:function(r){
            $('#tabla').load('tablaagregados.php');
          }
      });

    });
});
</script>