I want to have an N-by-2 grid like this, in which some elements (columns) might be set to display:none based on run-time factors, making the number of rows and colu
In your case, you can use last-child property. But you need to find the odd last element, because even last element no need to come in center place. So you can use it in the following way.
.col-xs-6
{
display:inline-block;
text-align:center;
}
.col-xs-6:nth-last-child(1):nth-child(odd) { /* This will find last child with odd element */
display:inline-block;
text-align:center;
width:100%;
}
DEMO