Real-world performance typically doesn't scale linearly with cores and threads.
From what I've seen, with constant frequency, same cache size per thread and same architecture like Zen 4, performance increases in various advanced, professional, useful workloads stack up approximately like this:
4 cores are 4x faster than 1
12 cores are 10x faster than 1
24 cores are 16x faster than 1
32 cores are 20x faster than 1
64 cores are 27x faster than 1
96 cores are 32.5x faster than 1
Of course, it doesn't look like that with everything. It's just an example, to illustrate how it can/might be. Higher clocks in lower corecounts (compared with higher corecounts) only make this non-linearity even more obvious.
Will 320, 384 or 512 cores be any useful, practical or economical outside some edge cases?
Theoretical flops/ops or Linpack flops ≠ actual performance (in finances, science and engineering for example). For AI there will be specialized hardware like there already increasingly is.
I'm always curious what the actual performance increases really are. Like, I currently think that Radeon 7800 XT is 12x faster than HD 5870 from Q3 2009, for about the same price when adjusted for inflation. :-)
By the way, Zen 4 can be even twice as fast as Zen 3 and Intel Sapphire Rapids (with the same number of cores and threads), largely thanks to its very decent architecture, large L3 cache, fast FPU and high memory bandwidth (333 GB/s for Threadripper and 461 GB/s for Epyc). Emerald Rapids is probably about 30% faster than Sapphire Rapids, so it's still behind AMD.
Global economy doubles in product every 15-20 years. Computer performance at a constant price doubles nowadays every 4 years on average. Livestock-as-food will globally stop being a thing by ~2050 (precision fermentation and more). Human stupidity, pride and depravity are the biggest problems of our world.