Thanks bbbt. So I have to divide my number of cities by 4... e.g., if I have 12 cities and 5 sources of Furs, then I can safely trade away 2 Furs? I wish the trade screen indicated it somehow.
Oh, and the actual location of the luxury doesn’t matter as long as it’s worked within my territory correct?
You can trade away 4 furs, preferably for 4 different luxuries. Each unique luxury provides +1 Amenities to 4 cities, with additional copies doing absolutely nothing. So 2 furs will be functionally the same as 1; you need, say, 1 furs and 1 gems to give +1 Amenities to 8 cities (it may be less than 8, as the game will do the optimization for you so if you have a particularly unhappy city it may receive +1 from furs and +1 from gems to make up for it).
Essentially, say you have cities A, B, C,... all the way to H. For non-luxury reasons city A is at -2 Amenities. When you get furs, +1 luxuries will be given to the four cities with the lowest amenities; in this case, A gets +1 to bring it to -1, and the others are tied; for simplicity's sake we'll say B, C, and D get the rest so that the totals are now:
A: -1
E, F, G, H: 0
B, C, D: 1
Then you get gems, which give +1 to your four lowest cities again. That would be A and three of the 0 amenity cities, say E, F, and G. This leaves you with:
A, H: 0
B, C, D, E, F, G: 1
Additional gems and furs will not do anything since you already have them; however, as in previous games, you can use extra copies to trade for luxuries you don't have, which would help (in this case, first to A and H, and then to the others).