Answer:
One square covers 100 square feet, so to cover 1900 square feet of roof you would need 19 squares. However, you also need "starter shingles" and "hip and ridge" shingles, plus you need to have a few extras because of waste from cutting angles.
Starter shingles go around the perimeter of the roof as a bottom layer for the first course of shingles. Hip and ridge shingles go, as you might expect, on the hips and ridges. In order to know for sure the exact quantity you would need for the entire roofing job you have to measure all of these areas.