As of writing, the most dense server CPU is the AMD EPYC 9965, which has 192 cores. It is possible to build a 2x CPU 1U server, so that is 384 cores per 1U. I am unsure what is the limiting factor: heat or space on the die (probably lots is just L1/2/3 cache). When you look at the interior of a vanilla 1U server, I don't think there is any remaining space to add more sockets (CPUs). Final note: SuperMicro also offers an Ampere ARM chip with 192 cores.