Use less when referring to a single item.|
Use fewer when referring to more than one item.
Less and Fewer
Writers often misuse the words less and fewer. The word less should be used for a single item (e.g., less time). Fewer should be used when there is more than one item (e.g., fewer mice).
Together we're using less bags
(should be fewer bags)
Less napkins. More plants.
(should be fewer napkins)
...fewer carrier bags
A SIMILAR TOPIC: AMOUNT AND NUMBER
Like less, the word amount should be used with a single item; whereas, the word number should be used when there is more than one.
See the lesson Amount, quantity and number.