I run a few linux HPC clusters, one particular platform runs on XCP-ng VMs using AMD epycs. We see a max of 64 vcpus recognised by the VM on XCP. You can assign more, but they are not visible from the VM OS. There also seems to be a total RAM size per VM too, which is 512GB, if memory serves (no pun intended ).
With regard to your other question - 7773X vs 7204 - on linux I would suspect that most codes would run the same binaries on both, but you may see a performance hit if an optimized binary wasn't compiled against one or the other cpu. But of course, there could just as easily be lots of other reasons for differences in performance across these boxes.