Many people mix up “fewer” and “less.” If you’re talking about the amount of something, you need to decide whether the item is all one thing or a group of many things. If it’s a group of many smaller things, you should use “fewer.” If it’s one thing, you should use “less.”
For example:
Incorrect: “The store was almost out of dog food. There were less cans on the shelves than there were yesterday.”
Correct: “The store was almost out of dog food. There were fewer cans on the shelves than there were yesterday.”