Fewer.
Recently I’ve started to think that these and other similar battles are lost.
It just feels so petty. Not a single person reading “less cops” was confused by its meaning. I get fighting against misuse of your/you’re, its/it’s, etc. because they can make things harder to read. Fewer and less, though, have the exact same underlying meaning (a reduction).
I’m something of a grammar Nazi, but just like I support letting “whom” die, “less” and “fewer” might as well just be interchangeable. There’s no loss of language utility in doing so, unlike “literally”'s tragic demise.
Me trying to get people to say they “are doing well” not “doing good” when asked “how are you doing?”
Ahh, I went on a rant about this, and someone already did it for me much more concisely.
https://www.merriam-webster.com/grammar/fewer-vs-less
Essentially, fewer is normally used for discrete numbers of things (e.g. “fewer apples”, “fewer boats”, or “fewer cops”) while less is used for amounts (e.g. “less water”, “less sand”, or “less money”).
As noted in the above link, there are exceptions. However, the exceptions listed are all with “than” or “or” added. Specifically, it’s pointing put that while “fewer items” is correct, “3 items or less” is also considered correct.
In the case of the sign, it is referring to the specific number of officers in the city, so it should use “fewer”. Does it matter? No, not really. Why did I bother saying anything? I got a chance to rep grammar and quote Stannis Baratheon at the same time.