Most of the games, especially old ones, don't have any effective multi-threading optimizations. It's quite sophisticated task to implement this kind of optimizations for games. It's possible to split some tasks of different type to a separated dedicated threads in order to distribute a workload to at least several processor's cores. But for another type of the tasks it's barely practically possible to do this at all. Otherwise, it would increase a cost (or a development time) of the final product significantly, which is unacceptable in the current reality where competition is very strong and everything is being developed in an ASAP mode.
There're a lot of games that have the most amount of workload processed only within 1 or 2 threads. For such games it doesn't matter how many cores a CPU has - 2 or 12, they will utilize only 1 or 2 anyway.
For this reason, i bet, there might be only a very few games in the world currently existing that would be capable to load a 12 or 16 core CPU to 100%, resulting in some insane number of frames per second, for example 500-1000 or whatever else.
Even more, majority of computer game players are more than enough satisfied of 60 FPS. Console players got used even to 30 FPS since long time ago. Game developers, of course, know this very well, so it's highly unlikely they would spend their extra resources in order to beat some limits that majority won't even notice, since it's more reasonable and profitable to redirect these resources to those parts that would make their software product more competitive on the market.
There're some ridiculous examples existing where a 30-years old games, having modern graphics and gameplay mods installed, in the best case are capable to run on the fastest modern CPU's only at average 100-200 FPS with drops to 30 FPS in a heavy graphics/physics situations. Just because these games use only a single core of the CPU.
As it's well known, there's a technological limit that makes it complicated to increase a single core performance further. So every next CPU generation nowadays has only a little increase in a single core performance. Most of the performance needs nowadays are filled by increasing a count of the cores in a CPU, which though makes no any sense for those games with partial multi-threading optimization or with no such optimizations at all.
Considering all this, if for some reason it is necessary to get as much frames per second as it is ever possible from some specific game, then most likely it's necessary to pick up a CPU that has the highest single core performance.
There're quite many reviews in the internet that show no any performance differences in many, even modern games, if they run on 6 or on 12 cores CPU's that have equal single-core performance.
You may want to inspect your game, for example, by a tool named Sysinternals ProcessExplorer in order to figure out how many threads the game runs, how it distributes the amount of workload between the threads, and finally to understand how many cores would be enough for that game.
As a result, it may appear that, for example with 6-cores Ryzen 5 5600X that game will give higher FPS than with 12-cores Ryzen 9 3900X.
Here is a list that may be used as a reference to compare a single-core performance of different CPU models: https://www.cpubenchmark.net/CPU_mega_page.html
The needed column in that list is named as "Thread Mark", clicking the header of that column makes the list sorted out by this value of the "Thread Mark".
In the "CPU Name" column you need to enter "AMD" or "Ryzen" to get rid of the mess caused by any unnecessary CPU names.
