Fewer and less

Counting against measuring.

The rule

Fewer counts separate things you could number. Less is for amounts you measure as a whole. Fewer cars, less traffic; fewer questions, less confusion.

If you can put a number in front of the noun, use fewer.
Money, time and distance take less even though they are countable: less than 20 minutes.

The same split runs through many and much, and through number and amount. Many mistakes, much confusion.