Tags on the floor plan mark each wall with a code, such as A1 or W3. A wall type legend or partition schedule explains each code.
One type may be metal studs with one layer of gypsum board each side. Another may be a rated shaft wall that runs to the deck.
A drywall estimator measures linear feet of each type and multiplies by height. Read the legend for height: to the ceiling, above it, or to the deck.
Check untagged walls. Ask which type applies. Fire-rated and sound-rated types cost more, so a wrong guess is expensive.