Though there is some contention about the validity of the rule, standard practice is to use "fewer" for countable items and "less" for an uncountable quantity. On Area 51, the "questions" beta site metric has been changed to "questions per day", with the following description:
15 questions per day on average is a healthy beta, 5 questions or less per day is worrying. A healthy site generates lots of good content to make sure users keep coming back.
Using the commonly accepted rule, "5 questions or less" should be changed to "5 questions or fewer" as questions are a countable quantity.