Wallpaper is the one material where the number printed on the package is reliably misleading. A double roll labelled 56 square feet yields around 32 in practice, and a large pattern repeat can drop that below 25.
Why yield is so much lower than the label
Strips are cut to wall height plus trim allowance at top and bottom. Anything left over at the end of the roll is shorter than a wall and therefore scrap.
On a 9 foot wall, a roll long enough for two strips wastes whatever remains, and there is nowhere to use it.
Pattern repeat is the second cut
A patterned paper has to line up at every seam. The longer the vertical repeat, the more you cut off each strip to bring the pattern into register.
A 24 inch repeat can cost you nearly two feet per strip. Random-match and free-match papers have no repeat penalty at all, which is why they are far more forgiving for a first project.
Order it all at once
Wallpaper has run numbers like tile has lot numbers, and colour shifts between runs are obvious on a wall.
Order the whole job plus an extra roll from the same run. The extra roll is cheap insurance and is what you will need in three years when something damages a strip.
Dye lots, and why one extra roll is not enough
Wallpaper is printed in batches, and every batch has a dye lot number on the label. Two rolls from different lots can differ in shade by an amount that is invisible in the shop and obvious on a wall.
This is why running short is worse than running short of paint. Paint can be re-tinted to match; wallpaper from a new lot cannot be made to match an old one.
Order every roll at once, from the same lot, and check the numbers on the labels when the order arrives rather than when you start hanging. Retailers do occasionally ship mixed lots.
Wall preparation decides whether it stays up
Wallpaper needs a sealed, smooth, sound surface. On bare drywall it needs a primer specifically made for wallpaper — one that seals the paper face so the paste does not soak in, and that will release the paper when it comes off years later.
Hanging paper directly on unprimed drywall is effectively gluing it to the paper facing of the board. Removing it later takes the drywall face with it, and that is a repair job across a whole room.
Glossy paint needs sanding first. Textured walls need skim coating. Neither step is optional, and both take longer than the hanging does.