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.