Maximum number of worker processes - Having multiple worker processes is similar to have multiple load-balanced servers, and is useful for similar reasons: in case one worker fails, the others can pick up the load. As such, there doesn't necessarily need to be more than one worker process, and there is no error generated when the worker processes are in use.
What could possibly be experienced if one worker process is not enough, is site slowness. This would possibly be due to long-running synchronous tasks that are causing the worker threads to block. I found useful information on this maximum number of worker processes (microsoft.com)
