12 items of *fewer* as it's a countable item

http://www.oxforddictionaries.com/words/less-or-fewer

docs/_includes/css/grid.html

@@ -347,7 +347,7 @@
 
 
   <h3 id="grid-nesting">Nesting columns</h3>
-  <p>To nest your content with the default grid, add a new <code>.row</code> and set of <code>.col-sm-*</code> columns within an existing <code>.col-sm-*</code> column. Nested rows should include a set of columns that add up to 12 or less (it is not required that you use all 12 available columns).</p>
+  <p>To nest your content with the default grid, add a new <code>.row</code> and set of <code>.col-sm-*</code> columns within an existing <code>.col-sm-*</code> column. Nested rows should include a set of columns that add up to 12 or fewer (it is not required that you use all 12 available columns).</p>
   <div class="row show-grid">
     <div class="col-sm-9">
       Level 1: .col-sm-9