Bootstrap is a 12 column rid, but you can put more than 12 columns in a row. The remaining columns will simply wrap onto the next line below, depending on the viewport.
In this example, on "md" viewports (≥992px), the contents would span 12 columns total (8 + 4). But on "xs" (<768px) the content would span 18 columns, there would be one full row (12 columns) and then below it a half-row (6 columns).
<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>
md...
| 8 | 4 |
xs...
| 12 |
| 6 |
EDIT: Make sure to check out the Responsive Column Reset section of the documentation if you run into any issues with columns not wrapping correctly.