I went back and listened to all three discussions regarding luxuries, at 22:00, 31:15 and 41:50, and I'm starting to think Stealth might be right (though it pains me to say so

). Particularly the bit at 31:15 seems to imply that additional luxuries don't get allocated to cities when >4.
Right, they say it would be hard to provide amenities for 20 cities, because your luxuries get spread thin. They probably would have mentioned additional copies at this point if it was a solution.
I really don't like this

but I can understand the advantages:
- It helps to make tall empires viable
- It counters ICS strategies (infinite city spam)
- makes trading less of a hassle (you don't have to keep track of the ever-changing number of copies you need - just keep one)
Still, it's tough to swallow for a realism lover like me...
"Hey man check out my Cotton Shirt, you should get one"
"Thats Lit man, though i cant

, Cotton is not available in my city"
"What!!, Dont we have the biggest Cotton Industry in the World?"
"Yeah but it is only allowed in 4 Cities, the rest need to be traded for a couple of pieces of gold and an umbrella because.......reasons"
"that is just stoopid"
Exactly
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Back to the initial topic of strategic resources:
I also believe that 2 copies of iron provide all the iron you need, any additional copy would only be useful for trading.
First of all, the tooltip with either 1 or 2 strategic ressources (depending on whether the city has an encampment) would not make sense otherwise.
If the additional iron would be permanently consumed, no one would ever want to build swordsmen in non-encampment cities - it would feel like a huge mistake, making you want to delete the miscreated unit ASAP (considering how scarce iron seems to be).
Secondly, the gameplay vids suggest that the average civ will only be able to grab 1-3 sources of iron with just 1 iron per source. This makes it even less likely that we'll need up to two iron per unit.
I firmly believe that this system applies:
- No iron, no special units.
- 1 source: you may produce them, but only in cities with encampment (representing rare elite units)
- 2 sources: you have a strong iron industry and can afford to equip large parts of your army with it.
- 3+ sources: your iron industry is so strong that you may even trade it away while still fielding lots of well-armed units.
This would make the bonus some civs get (UU's don't need a strategic resource) more reasonable balance-wise.
It would also help making trade simpler to understand (keep one of each luxury and two of each strategic resource, the rest is for trading).