Does the average American family have less than two kids or fewer than two kids?
How to Remember It:
Fewer refers to things that can be counted (fewer kids, fewer chairs). Less usually refers to quantities of things that can't be counted (less coffee, less agitation).
However, under certain circumstances less, not fewer, is more commonly used with countable things. For example: Less than twenty miles, less than five dollars, and 1500 words or less, are considered standard.
As for the express lane at the supermarket, "ten items or fewer" follows the general rule, but "ten items or less" is also widely accepted and more often used.