Have you ever done a small crown procedure? Around 300-500 grafts?

It all depends on the extent of the area to be covered. The number of grafts that you describe would be able to cover a very small area of about 4x4 cm. Apart from this the expected density is not going to be as much as the density that we obtain when we implant the anterior part of the skull. The most important in your case is to have realistic expectations because usually men asking for correction of such small areas are not satisfied with anything less than a very high density.

Hair loss has many causes, including stress, medications, autoimmune, genetic, trauma, etc. Medical treatments can work and options would include finasteride (propecia), minoxidil (rogaine), PRP (platelet rich plasma), laser light therapy, and vitamins. It is often uncertain which medical therapy will help which patient and so a combination of therapies is typically recommended to maximize benefit. A hair transplant is a surgical option and is appropriate for certain conditions and individuals. 
The goal of a hair transplant is to provide fuller coverage while preserving a natural appearance. An assessment takes into account such factors as the apparent thickness and curl of your donor hairs, the color contrast between your scalp and hair, your hair loss pattern, your age, and many other factors. The state of the art technique is FUE (follicular unit extraction), performed with a hybrid blunt punch that minimizes graft transection while allowing for the use of the smallest possible punches to minimize donor site scarring while maximizing regrowth.
In your situation, 300 - 500 grafts is usually not enough for a noticeable difference in the crown. Usually, you would need at least 800 to make any sort of difference. 
Gary Linkov, MD
Hair Specialist
Manhattan

Addressing the crown with hair transplantation as a stand-alone procedure can be very underwhelming for a patient.  The crown is a difficult area to achieve the appearance of adequate density.  IT is not as much the amount of grafts that factor into the appearance of density but rather the other factors which can limit the appearance of adequate density to the crown.  The radial growth of hair in the region, the convex shape of the scalp, and the directional growth of hair all limit the appearance of density.  In general, frontal hair loss and transplantation to the frontal area of the scalp is able to achieve a higher appearance of density.  Combined measures such as medical therapy, scalp micropigmentation, and hair transplantation will maximize the results of crown hair loss.