A queue is not equal to a processor core.
The purpose of GCD is map queues to cores appropriately. Appropriately means not just the number of cores, but also taking into consideration total system workload.
Just create queues as needed and let GCD do the management. Having more queues than cores is the same as having more processes than CPUs, the operating system will manage it for you.
Don't be naive, though. Do create sequential and parallel queue intelligently for the tasks at hand. But the decision should be based around the nature of the tasks, not the user's environment.