What matters is that the server app uses as much CPU as it can when it needs it.

With a sync server, a worker is inactive as long as it is waiting on IO, so you need enough workers to maximize the chance that all workers are busy, else, some clients are waiting even if you've got the CPU to deal with them.

With an async server, a single worker handles many clients simultaneously, in theory, a single worker per core is sufficient to eat all the CPU available.