Language purists like to harp on certain problems. One of those problems being the distinction between "less" and "fewer." According to such purists, that line in the supermarket should not say "10 items or less" but "10 items or fewer." Well, I say phooey to that. Less is intended as a counter for singular objects, yes, but it has also proven its use and value as a counter in relation to numbers that are continuous (such as miles per hour, or age). Do we say "She is fewer than 21 years old, so she can not drink." No, of course not! It sounds pompous and foolish. "One less car" also sounds a lot better than "fewer than one car." "Less" may often be considered idiomatic in it's usage (Describe him in less than 20 words) if that number is considered a standard. If both "less" and "fewer" are both available, such as "Less/fewer than 20 of the students voted", "fewer" is the better choice because it enhances vividness and concreteness. But that does not mean that "less" is a grammatical error! So go ahead and use whichever one has always felt more natural: it's probably fine!