When buying wallpaper it is to make sure that all the rolls you buy are the same shade / batch number, this is because slight colour changes between shades / batches may not be visible to the naked eye until you have hung the paper on the wall and it has fully dried, then you are faced with the task of taking off pieces of the wallpaper and finding more wallpaper with the same shade / batch as the rest to replace it, or starting again, to avoid this happening it is best to know how many rolls you are going to require for the complete job before starting.