How much grafts I need to cover this area? (Photo)

There are a couple of variables at play here - your hair density, follicular diameter and natural scatter, for example.  In your case, a rough estimate would be between 1500 and as many as 2000.  But you could go down to 1250 or so depending on natural density, etc.  The range can be quite wide, and a good physical exam should be able to give your doc an idea after measurements.

Hair transplant surgery is not always about "how many grafts" you need.  What happens if you continue to lose more hair behind the hair line?

Thanks for your photo and based on it you will need around 2000-2300 grafts to reconstruct the frontal part which means filling in the temples and lowering your hair line a bit. This amount will help to achieve a natural and good result with high density.