Side by side in Bootstrap, jumping line

The sum of all cols must always be 12 , for example:

• If you have 4 divs with class .col-*-3 then the sum will be 12 (3 + 3 + 3 + 3 = 12)
• If you have 3 divs with class .col-*-4 then the sum will be 12 (4 + 4 + 4 = 12)
• You can also do .col-*-6 + .col-*-3 + .col-*-3 for example (6 + 3 + 3 = 12)

You <div class="col-md-2" style="margin:0;margin-bottom:3%;margin-top:2%;padding:0.1%;"> within col-md-12 , as I said, col- should always go inside row , so it's wrong.

You have <div class="col-md-12" style="margin:0;padding:0;"> within col-md-2

You have two <div class="col-md-6"> within col-md-12

The <a> tag is badly closed: <a href="filme.php?id=<?php echo "$linha[id]";?>"> , you have placed </a> inside another tag, probably not to conflict with the youtube link. But this is wrong.

It has two col mixed with other HTML elements that are not part of the grid.

Never change the borders of a col- using margin: or margin-left or margin-right , I think top and bottom have no problem. li> Although it does not affect the grids you have an error in your CSS is wrong h2 style="text-align:left; color:white; margin:-top:1%; , has margin:-top , correct is margin-top:1%;

How to solve

As I explained here link being PHP you can use something like $i % (numero de colunas desejada), no seu caso você usou "col-md-2" so I think you want 6 columns per lines

It should look like this:

<div class="row">
    <div class="col-md-12"> <!-- aqui está certo total é igual 12 -->
        <h2 style="text-align:left; color:white; margin:-top:1%;">Ação/Aventura</h2>

        <?php
        require 'conexao.php';

        $consulta = $PDO->query("SELECT * FROM filmes WHERE categoria1 = 'acaoaventura' ORDER BY id ASC;");

        while ($linha = $consulta->fetch(PDO::FETCH_ASSOC)) {
        $i = 0;

        while ($i < 20) {

        $needarow = !($i % 6);
        $i++;//Deve vir depois
        ?>

        <?php if ($needarow) { ?>
        <?php if ($i > 1) { ?>

        </div> <!-- //fecha .row -->

        <?php } ?>

        <div class="row">

        <?php }?>

            <div class="col-md-2" style="margin-bottom:3%;margin-top:2%;padding:0.1%;">

                <!--//acho que nem precisa de col aqui -->
                <a href="filme.php?id=<?php echo $linha['id'];?>">

                    <img src="" class="img-fluid">
                    <p style="color:yellow; text-align:center; margin:0; padding:0;"><?php echo $linha['nome']; ?></p>
                </a> <!-- Link fechado -->

                <div class="row"> <!--// Row necessário para adicionar -->
                    <div class="col-md-6">
                        <p>16 visitas</p>
                    </div>

                    <div class="col-md-6">
                        <a class="venobox_custom" data-type="youtube" href="https://youtu.be/<?php echo $linha[trailer]; ?>?autoplay=1"><p><i class="fa fa-film" aria-hidden="true"></i> Trailer</p></a>
                    </div>
                </div>

            </div>

        <?php
        }   //Termina o loop
        ?>

        </div> <!-- //fecha o ultimo .row -->
    </div>
</div>