How much siding do I need?

Add up the area of every elevation, subtract the windows and doors, then add about 10% for cuts. A house with 1,500 ft² of wall and 150 ft² of openings needs 1,350 × 1.10 = about 1,485 ft², or roughly 15 squares.

100 square feet. Siding, roofing and some other exterior materials are all quoted this way, so a supplier saying "15 squares" means 1,500 square feet of coverage. Boxes are then whatever fraction of a square that particular product ships in — commonly two squares per box for vinyl.

Around 10% on a simple rectangular elevation. Go higher — 15% or more — where there are lots of gables, dormers, corners or angled cuts, because every angle produces an offcut you cannot use. Lap siding on a complex facade wastes noticeably more than flat panels.

Deduct the large ones, yes. Many estimators leave small openings in as a built-in waste allowance, on the grounds that the cuts around a small window waste nearly as much material as the window saves. Either convention works as long as you are consistent between the takeoff and the quote.