You hear and read sentences like this all the time: “Matilda has less dogs than she did last year.”
Matilda can count her dogs. For items you can count, use FEWER: “Matilda has fewer dogs than she did last year because she gave three puppies to her nephew.”
Use LESS for things you can’t count: “Less milk has been sold since the prices increased so drastically.” You can count glasses of milk; for that you would use FEWER, but for milk in general, use LESS.
You’ll notice that most checkout lines in markets have signs saying “10 ITEMS OR LESS.” On the rare occasions I’ve seen that sign written correctly, I feel as if I am going to faint (from joy and amazement).