To answer your first question ... it depends. You can have one worker per task or you can open a pool. There are examples of both in the "batch" documentation.
For your second question, if you exceed the amount of workers, the jobs will queue up and process as previous work finishes. You can monitor this using the Job Monitor, found under the "Parallel" menu.
You might also want to take a look at the jobs and tasks functionality if you need more control.
