How to make the select complete based on the selected variable in another select

I need to make sure that after choosing the product, in the quantity selection, display a list with the values according to the quantity available, for example: if you have 10 pieces of a trouser, display values from 1 to 10 in the select field automatically.

    <form name="products" id="products" method ="post" action ="registering_request.php">
        <fieldset style = "width: 640px; height: 640px; margin: 0px auto; border-size = 0px;"><legend align ="center"><h2>Novo Pedido</h2></legend> 
        <br>    
        <select name="CmbCustomers" id="CmbCustomers"> 
        <option value="">Selecione o Cliente</option>
        <?php
        $query_cust = "SELECT 'name' FROM customers";
        $select_customers = $PDO->prepare($query_cust);
        $select_customers->execute();

        //Conta a quantidade de linhas de clientes no BD;
        $number_of_rows_cust = $select_customers->rowCount();

        //Pega os dados dos clientes no BD; 
        $data_cust = $select_customers->fetch();

        //Verifica se a quantidade de linhas de clientes no BD é maior que 0; 
        if ($number_of_rows_cust > 0){
            echo '<option value="'.$data_cust['name'].'">'.$data_cust['name'].'</option>';
            } else {
            echo '<option value="">Sem clientes cadastrados</option>';
        }
        ?>
        </select>
        </p>
        <select name="CmbProd1" id="CmbProd1">
        <option value=""> Selecione o Produto </option>
        <?php
        $query_prod1 = "SELECT 'name', 'id' FROM 'products'";
        $select_product1 = $PDO->prepare($query_prod1);
        $select_product1->execute();

        //Conta quantas linhas tem no banco de dados; 
        $number_of_rows_prod1 = $select_product1->rowCount();

        //Pega o conteudo do banco de dados;
        $data_prod1 = $select_product1->fetch();

        //Preenche o campo option com o conteudo do BD;
        if ($number_of_rows_prod1 > 0){
            echo '<option value="'.$data_prod1['id'].'">'.$data_prod1['name'].'</option>';
            } else {
            echo '<option value="">Sem produtos disponíveis</option>';
        }
        ?>
        </select>
        <select id="CmbQuantProd1" style="display:none"></select>
        <script>
            $("#CmbProd1").on("change",function(){
                var id_product = $("#CmbProd1").val();
                $.ajax({
                    url: "verifyQuantity.php",
                    type: "POST",
                    data:{id_product:id_product},
                    beforeSend: function(){
                        $("#CmbQuantProd1").css({"display":"block"});
                        $("#CmbQuantProd1").html("Carregando...");
                        },
                        success: function(data)
                        {
                            $("#CmbQuantProd1").css({"display":"block"});
                            $("#CmbQuantProd1").html(data);
                        },
                        error: function(data)
                        {
                            $("#CmbQuantProd1").css({"display":"block"});
                            $("#CmbQuantProd1").html("Houve um erro ao carregar");
                        }
                    });
            });
        </script>
        </p><br><br><br>
        <center>
        <button type="submit" name="salvar" value="salvar" class="bVerde">Registrar</button> &nbsp &nbsp <button type="button" name="voltar" value="main.php" class="bCinza" onclick="window.location='main.php';">Voltar</button></p>
        </center>   
        </div>
    </body>

And in verifyQuantity.php:

    <?php
    include 'conexao.php';
        $sql = $conn->prepare("SELECT * FROM stock WHERE id_product = '".$_POST['id_product']."'");
        $sql->execute();
        $fetchAll = $sql->fetchAll();
        foreach($fetchAll as $result)
        {
            echo '<option value="'.$result['quantity'].'">'.$result['quantity'].'</option>';
        }
    ?>

That's how it worked.