Yes, a worker is limited to a single core of usage, it cannot use up two cores for instance; We PipeTransport to use up more workers.
As for worker count, you can run as many as the core count or more there should not be a recommendation outside of ensure you run enough workers. You could have many reasons to want to run more workers if say you were playing with server limits to ensure piping.
You could run the workers lighter and if they beef up, close other workers and reload them when CPU is below 50%