A unique Luxury provides 4 amenities, limited to 1 per city -- full stop, end of story. Extra copies of a luxury provide no more amenities, no matter how many cities you have. Extra luxuries are only good for two things: (1) to be traded to another civ in exchange for whatever you might want or need (including perhaps another unique luxury that will provide 4 amenities) or (2) to serve as back-ups or hedges for the risk that your first copy of that luxury might get pillaged, or be located in a city that is conquered by another civ, or otherwise lost through some unimaginable calamity. During the pre-release period, the community convinced itself (on zero evidence, but oodles of wishful thinking) that a second copy of a luxury could provide 4 more amenities (for cities 5 through 8), that a third copy could provide another 4 amenities (for cities 9 through 12), etc. All of that was incorrect, and remains incorrect.