Use the adjective fewer to describe countable items; otherwise, use the adjective less. And here’s a tip: in general, if the noun is plural, use fewer; if it’s singular, use less:

Fewer treadmills line the floor of the gym. 

Less equipment lines the floor of the gym. 

In the first example above, treadmills can be counted. The term is plural and takes a plural verb: line. In the second example, equipment cannot be counted. One does not say, “one equipment, two equipment.” The term is singular and takes a singular verb: lines.

But some plural nouns—especially ones referring to money, distance, or time—are thought of as singular ideas and take a singular verb:

Fifty dollars is a lot of money.

One hundred miles is far to travel.

Thirty minutes is the average length of a sitcom.

In these cases, use less:

She earns less than fifty dollars per hour.

 We have less than one hundred miles left on our journey.

 The show is less than thirty minutes.

Less is also generally used with percentages expressed as x percent of y, even when the verb in the sentence is plural:

Less than ten percent of staff members work from home.

In the example above, the plural term staff members takes a plural verb, work, but less refers to the singular term percent.