Oh, this has been a bone of contention in proscriptive circles for quite a while.
The "use fewer only for countables and less only for non-countables" debate has raged ridiculously and futilely in the background, while speakers and writers of English have completely ignored it.
The discussion arose because fewer can't be used for non-countables, so a number of (very loud) people got the idea that less should only be used for non-countables, to balance it.
That's all very nice, and all very logical, but unfortunately it's nonsense, because less has always been used in every native dialect for both countables and non-countables, and wishing that it weren't won't make it stop.
-- If the person paying you to write or marking your work insists on your following this matter of style, follow it.
-- If your writing isn't being paid or marked, use the one that feels best to you in the sentence.
-- If someone challenges your (unpaid) usage of less for a countable, ask him if he truly believes that twelve is not less than twenty.