How many grafts would I need approximately for an even hairline? (Photo)

Photos cannot assess if you would be a "great" candidate for surgery.  Usually this involves seeing a doctor in person so they can understand your goals and expectations.  You need to be clear on this and that may be why you get a wide range from 700 to 3000 grafts.  One surgery alone may not give you full (normal hair) density.

To bring your hairline down to the extent you have drawn out in your photo, 2,000 grafts would be about right to give you the density that would look natural. The grafts must be single hair grafts for the hairline, thus requiring the suggested 2,000 grafts.
I hope this has helped you. Thank you for you question. Be sure to choose someone with lots of experience and skill. Best of luck to you.

The number of grafts needed will essentially depend on how much you will decide to lower your hairline, on your hair caliber therefore on the implantation density rate. Normally, to achieve a natural looking feminine hairline restoration the implantation density must be quite high. Based on your picture, I would recommend you to get around 2500-3000 grafts via FUT (preferable for women) or FUE.  

The area you outlined may take between 1200-1500 grafts to recreate the hairline you want. Take a look at the pictures below