A 2,500 ft² roof equals 25 squares. Three bundles of architectural shingles typically make one square (33 ft² per bundle). Check the bundle wrapper for the specific coverage of your product.
Squares measure the actual roof surface area, not the building footprint. Multiply the building footprint (plan area) by the pitch multiplier to get the surface area in square feet, then divide by 100 to get squares.
When you order shingles, you order whole squares (or bundles); always round up after adding waste. A 25-square roof with 10% waste = 27.5, round up to 28 squares or 84 bundles.