The following is taken from Good and Bad English by Whitten and Whitaker:
Less appertains to degree, quantity or extent; fewer to number. Thus, less outlay, fewer expenses; less help, fewer helpers; less milk, fewer eggs.
But although few applies to number do not join it to the word itself: a fewer number is incorrect; say a smaller number.
Less takes a singular noun, fewer a plural noun; thus, less opportunity, fewer opportunities.