Ah, *less* vs. *fewer*. Another arrow in the prescriptivist's quiver of pointless pedantry.

There's even a [Wikipedia article about the dispute][1]. There is also a [Language Log entry][2] about the matter too.

According to [Merriam-Webster's Dictionary of English Usage][3], a usage guide that looks carefully at the history of usage advice, the rule creating a clear separation for *less* and *fewer* was invented in 1770 by Rober Baker in his book _Reflections on the English Language_, where he wrote in a comment on *less*:

> The Word is most commonly used in speaking of a Number; where I should think *Fewer* would do better. *No Fewer than a Hundred* appears to me not only more elegant than *No less than a Hundred*, but more strictly proper.

The Merriam-Webster Dictionary of English Usage authors then comment:

> Baker's remarks about *fewer* express clearly and modestly—“I should think,”, “appears to me”—his own taste and preference. It is instructive to compare Baker with one of the most recent college handbooks in our collection:
>>*Fewer* refers to quantities that can be counted individually.… *Less* is used for collective quantities that are not counted individually… and for abstract characteristics. —Trimmer & McCrimmon 1988

> Notice how Baker's preference has here been generalized and elevated to an absolute status, and his notice of contrary usage has been omitted. This approach is quite common in handbooks and schoolbooks; many pedagogues seem reluctant to share the often complicated facts about English with their students.

> How Baker's opinion came to be an inviolable rule, we do not know. But we do know that many people believe it is such.

They then give many examples of usage of less for countable quantities, and add finally:

>The examples above show native speakers and writers of English using *less* of count nouns in various constructions. *Fewer* could have been used in many of them—at times it might have been more elegant, as Robert Baker thought—but in others no native speaker would use anything but *less*.

With regards to the example in OP's question, either *fewer* or *less* would be perfectly grammatical, but so many people are under the spell of the rule that *less* must never be used with countable nouns that anyone who doesn't follow rule may be subject to criticism.


  [1]: http://en.wikipedia.org/wiki/Fewer_vs._less
  [2]: http://itre.cis.upenn.edu/~myl/languagelog/archives/003775.html
  [3]: http://books.google.com/books?id=2yJusP0vrdgC&lpg=PA592&pg=PA593#v=onepage&q&f=false