Bootstrap's grid columns number per row

Question

Bootstrap comes with a 12 columns grid system that must be placed within rows.

My question is, does the column number / row must be 12 (or less), or can I have a layout like the following

<div class="row">
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
</div>

My understanding was that columns number within a row mustn't exceed 12, so based on my previous snippet, I would have made something like this

<div class="row">
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
</div>
<div class="row">
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
  <div class="col-md-4"></div>
</div>

Is there anything I missed ?

Answer 1

Bootstrap allows you to "stack columns", the following is taken from their docs:

<!-- Stack the columns on mobile by making one full-width and the other half-width -->
<div class="row">
  <div class="col-xs-12 col-md-8">.col-xs-12 .col-md-8</div>
  <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
</div>

<!-- Columns start at 50% wide on mobile and bump up to 33.3% wide on desktop -->
<div class="row">
  <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
  <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
  <div class="col-xs-6 col-md-4">.col-xs-6 .col-md-4</div>
</div>

Also from their docs:

With the four tiers of grids available you're bound to run into issues where, at certain breakpoints, your columns don't clear quite right as one is taller than the other. To fix that, use a combination of a .clearfix and our responsive utility classes.

<div class="row">
  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>

  <!-- Add the extra clearfix for only the required viewport -->
  <div class="clearfix visible-xs"></div>

  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
</div>

Here, the col-xs values add up to 24, with a clearfix adding the required break