counting.css

.box{
	width: 100px;
	height: 100px;
	float: left;
	margin: 10px;
	border: 1px solid black;
}


/* Only one item */
.box:only-child{
  border-width: 10px;
}

/* If more than one */
.box:not(:only-child){
  border-width: 10px;
}

/* Only one of type */
div:only-of-type{
  border-width: 10px;
}

/* Exactly n items (6 here ) */
/* nth-last-child mathches nth element from last */
.box:nth-last-child(6):first-child, 
.box:nth-last-child(6):first-child ~ .box{
  border-width: 10px;
}

/* the 6th from last up */
.box:nth-last-child(n + 6){
  border-width: 10px;
}

/* Matches 6 or more items */
.box:nth-last-child(n + 6), 
.box:nth-last-child(n + 6) ~ .box{
  border-width: 10px;
}

/* Matches 6 or less items */
.box:nth-last-child(-n+6):first-child, 
.box:nth-last-child(-n+6) ~ .box{
  border-width: 10px;
}