“I think they mean twelve items or FEWER!” my mom would say loudly, as I ducked my head in feigned embarrassment. I’m sure I hissed back something like, “Mom, no one cares,” while at the same time absorbing the grammar guideline she was emphasizing. Now, I always notice when the fewer vs. less rule is broken, though I don’t get nearly as irritated by it as she did back in the day.
The guideline is pretty simple: if the amount you’re describing is quantifiable (i.e. something that can be counted), use “fewer.” If it’s not, use “less.”
For example, my grandpa has less hair than I do. I own fewer hats than he has in his top drawer alone. The number of hats we both own is quantifiable; the amount of hair we have is not (unless we’re talking about individual hairs – but I wasn’t).
So, it’s simple. Or… maybe not. Check out this example from the Chicago Manual of Style site to see how it’s not always as easy as being able to count or not. Using “less” instead of “fewer” is very common, as our check-out lanes at the grocery store show. It’s not a stretch to think that the distinction between these terms will become interchangeable in time, much to the horror of the purists who forget or fail to realize that our grammar rules shift and change with our language usage over time. We certainly understand the signs that say “12 items or less” – i.e. it’s not that others won’t be able to understand you if you flip-flop the terms.
In my opinion, follow the formal rule in formal writing. In common usage, you’ll probably be able to get away with using either term, whenever. But when something is really quantifiable and you say “less,” like, say, “You had less pieces of cake than I expected. Please have another.”, I am probably going wrinkle my nose and say, “FEWER!”