Less or Fewer

Less is used with uncounted nouns: less soup, less intelligence, less foliage.
Fewer is used with countable nouns: fewer voters, fewer apples, fewer commercials.

Examples:
I cooked less meatloaf this time because I didn’t want too many leftovers.
He has less strength in his left hand.

This box contains fewer pencils than that one.
I earned fewer points in the game than Jonas did.