The capacity for supervision is limited. There is no rule how many students should be supervised by one professor. Talk to your professor if he/she still has the capacity to supervise you. You will see that some of the colleagues are already fully booked long in advance. Asking on the last day before your application is due is somehow the worst idea in this field.
Remember that the professor might be in his/her research semester the semester you want to use the service of supervision. If you take care of things early this should not cause problems. If you ask late, you will definitely have trouble in finding a supervisor.
Also respect that your professor may not personally available the moment you are in need of a physical signature. Plan ahead to avoid all those obstacles.
