I would think that a man's facial hair would be just like the hair on his head in respect to having a finite number of hair follicles.  Once full adulthood is reached, there probably won't be any way to develop more follicles.  Some guys just really don't have much facial hair, and others just have slow-growing hair.  Just in case your hair is a little slower to grow, I'd hold on a few more weeks to allow it to grow as much as possible.