Trading luxury resources - basic question

[Sherlock]

In CiV you could trade any luxury item you had more than one of. If you had one you were getting all the benefit (happiness) you were going to get. And that was across your whole civilization, not just one city.

But in CiVI don't luxury benefits (amenities) need to go across several cities? And if you have more cities then you need a higher number of that luxury item to keep all the cities with the benefit?

So how do you know what number of luxury items you can trade? Do you have to figure it out by the number of cities you have, or the population of those cities or what?

Any basic explanation of this will be appreciated.

 

[PYITE]

In CiV you could trade any luxury item you had more than one of. If you had one you were getting all the benefit (happiness) you were going to get. And that was across your whole civilization, not just one city.

But in CiVI don't luxury benefits (amenities) need to go across several cities? And if you have more cities then you need a higher number of that luxury item to keep all the cities with the benefit?

So how do you know what number of luxury items you can trade? Do you have to figure it out by the number of cities you have, or the population of those cities or what?

Any basic explanation of this will be appreciated.

Each luxury resource gives 1 amenity to 4 cities. (Aztec - 6 cities)
You aren't in control of how these are distributed to your cities; the game engine is.
Multiple copies of the same one doesn't increase your total amenities
Trading the extra is the only benefit that it can give you.
The trade screen shows you how many copies of each you have.
The AI only asks you to trade ones that you have an extra copy of.
You need 2 copies of a strategic resource to build the corresponding unit in a city without a barracks. The AI will ask you to trade the 2nd.