There is nothing wrong with using more than 12 column units in .row, and in fact the Bootstrap docs state:
“If more than 12 columns are placed within a single row, each group of extra columns will, as one unit, wrap onto a new line”
There are also examples in the docs that demonstrate why this “column wrapping” in necessary: https://getbootstrap.com/docs/3.3/css/#grid-example-wrapping. It’s ok to have more that 12 units in a single .row. tag, just remember you may need to use responsive resets. 12 units is a limit of the visual row (horizontally across the viewport), but not necessarily a .row div which is simply a grouping of columns.
More than 12 column units inside a single row enables a layout to be responsive.
Read more about why more than 12 in a row is often necessary.