Your area and tile size in — exact tiles needed and how many full boxes to buy, with a waste allowance for cuts and breakages.
Tiles break in transit, get cut to fit around edges and fixtures, and need to be discarded if a pattern doesn't line up cleanly. A straightforward rectangular room laid in a simple grid can often get away with 10% waste; diagonal layouts, herringbone patterns, or rooms with lots of pipework and fixtures to cut around should budget 15-20% instead.
Tile boxes can't be split, and batch numbers (the dye lot) can vary slightly between production runs — buying all your tiles from the same batch in one purchase avoids visible colour variation partway through a job. It's also worth keeping a spare box after the job for future repairs, since matching an exact discontinued tile years later is often impossible.
A 12 m² bathroom floor with 300×300mm tiles (11 per box) and a 10% waste allowance needs 147 tiles, which rounds up to 14 full boxes — 154 tiles total, 7 more than the strict minimum.