Estimating sheets from area
Drywall is estimated by area, so the key step is adding up every surface you plan to cover, including ceilings. Dividing that total by the coverage of a single sheet gives the raw count, and adding a waste allowance accounts for the offcuts created when fitting around corners, openings and fixtures.
Choosing sheet sizes and finishing
Fewer, larger sheets mean fewer seams to tape and finish, which can speed up the job on long, unbroken walls, but they are heavier and harder to maneuver in tight spaces. Whatever size you choose, budget for screws, joint compound, tape and corner bead, and plan for several coats of compound with sanding between them.