AP Style holds to the traditional distinction between “fewer” and “less.” This rule states that, in general, you should use “fewer” for individual items, and “less” for bulk or quantity. Below are a few examples,
Wrong: The trend is toward more machines and less people. (People in this sense refers to individuals.)
Wrong: She was fewer than 60 years old. (Years in this sense refers to a period of time, not individual year.)
Right: Fewer than 10 applicants applied. (Individuals).
Right: I had less than $50 in my pocket. (An amount.)
Right: I had fewer than 50 $1 bills in my pocket. (Individual items.)