On the news last night I heard both newscasters and interviewees complain about Oakland’s new policy of two-man police cars which will have the effect of reducing the number of cars on patrol. They all said there would be “less police” on the street, or “less patrol cars.” No, there will be fewer cars, not less. As for police, there will be just as many, but they will be in fewer cars. The rule is simple: if what you are talking about is in discrete, countable units, use the word fewer. If it’s not, use the word less. Less police presence is okay because “presence” is not countable. Fewer cars. Less poverty. Fewer poor people. Less suffering. In the opposite direction it’s easier: it’s always “more.” More cars, more poverty, more suffering. Less/fewer parallels the use of “much” and “many”. If you would ask the question “How much” about something, then less is the right word. If you would ask “how many” then use fewer.
It can be tricky for some words like money. Money comes in countable units, but it is the individual units that are countable, not the concept of money in general. You wouldn’t ask “how many money do you have?” So you should say “less money,” not “fewer money” or “fewer monies.” You would say “How many dollar bills do you have?” so it is proper to say “fewer dollar bills.” The units are countable, but the generalization is not.