Although the more closely spaced your studs, the more screws you'll be using to attach drywall panels, you can still figure on an average of one screw for every square foot of drywall. Say your ceiling is 18 feet by 18 feet (5.47 meters by 5.47 meters); this means it's 324 square feet (30.10 square meters). Since a pound (0.45 kilograms) of screws contains about 320 screws, you can get away with a pound of screws for this job.
When you're dealing with drywall, it's a bit more complicated, as you also have to deal with windows and doors. Although you might not be able to subtract the window and door areas from the number of drywall panels you buy, you can subtract the number of screws that would otherwise be in the area occupied by the windows and doors. To know the area you'll be working with, measure the height of the room and the width of each wall. Multiply the height by the width of each wall and add together the total surface area of each.
Say you have a room with walls that are 8 feet (2.4 meters) tall; two of the walls are 14 feet (4.2 meters) long, and the other two walls are 12 feet (3.6 meters) long. The area of each of two of the walls is 8 feet (2.4 meters) by 14 feet (4.2 meters), which equals 112 square feet (10.4 square meters), while the area of each of the other two walls is 8 feet (2.4 meters) by 12 feet (4.2 meters), which equals 96 square feet (8.9 square meters). The ceiling is 12 feet (3.6 meters) by 14 feet (4.2 meters), which equals 168 square feet (15.6 square meters). The total square footage of the room is 584 feet (54.3 square meters), which equals 96 square feet (8.9 square meters) times two, plus 112 square feet (10.4 square meters) times two, plus 168 square feet (15.6 square meters).
If you use 1 5/8-inch (4.1 centimeters) screws, you'll end up with around 150 screws in a pound, so figure on a 4-pound (1.8 kilograms) box of screws taking care of the room, with some left over. Actually, a 5-pound (2.3 kilograms) box of screws will usually be enough for any room, unless it's really large. If you're not installing drywall on the ceiling, you'll need fewer screws, and if you decide to add partitions, you'll need more screws.