It's quite easy to calculate. 1 luxury = 1 amenity for 4 cities. Based on the numbers of cities you have you shouldn't have problem to calculate. 8 cities you can use 2 luxuries of same type - rest is unused - sell it. Next thing you go to reports and you see status of your cities - based on do you want them happy or estatic for bonuses you trade even those that are used but you don't need all that bonuses but fast cash.
My understanding was that having 2 copies of a luxury resource did not provide an amenity for 8 cities. I read that somewhere but have no way of confirming it because there is no explanation for the amenities that I am getting credit for.
However, what you mentioned makes total sense and is how it should be in my opinion.
I'm almost 100% this is how it is. The only thing that can confuse you is that there is system behind the scenes that appoints those amenities based on housing and size of cities, etc... so sometimes it can look the numbers don't add up somewhere ... also when you go into negative amenity, war or whatever, it can take some turns to level the real amenity state ... and similar quirks tends to confuse ... but one luxury can make 4 cities happy, that's 100%.
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.