Possesive apostrophe

There something about this that I’d like to learn clearly,
when and when not to use it. Here’s a an example of what I’m trying to convey:

[color=blue]I heard a new York’s police’s conversation.


[color=blue]I heard a New York police’s conversation.

I know there might be other situations like this but right now it just slips my mind.

Is there any rule I can observe? or anything that could be of help, I’ll apreciate it. :?:

[color=blue]Let me tell you all that I found the answer to my question:
It has to do with the attributive. It is that a noun can function as an adjective thus modifying the main noun.
When used in this way is called attributive noun and may also be a compound noun and a phrase noun. In such case, the attributive noun is traditionally hyphenated.

In the case of attributive noun phrases that include a quantifier (numbers), American usage follows an interesting rule: when the noun phrase is transform into a modifier,the noun component of the original phrase is put into singular form.
Example:the noun phrase “two cars” become the adjective “two-car,” as in “two-car garage” while in British would be “two cars garage” ( at least formaly).