How many grafts would I need for a second surgery to increase density? (Photos)

I did not see any mention of your age,which is quite important. Typically in younger men (less than around 35) we are hesitant about transplanting in the crown and back as these areas tend to expand in pattern of loss and require many grafts.The other important factor is whether you are using any adjuncts to prevent further hair loss-e.g. Propecia (Finasteride),Rogaine (Minoxidil), Low Level Laser Light Therapy and/or PRP (Platelet Rich Plasma).

A great first step would be an online consultation. Often you can submit photos to top hair restoration surgeons for a complimentary evaluation. This should help lead you in the right direction. Having multiple consultations with different doctors cannot hurt. 

A repeat of a procedure the same size would be warranted; however, be careful that if you are going the FUE route, that you will not exceed the safe limit before getting donor site balding which for most people is about 2600 FUE grafts