The first things you need are accurate measurements of the walls that are to be covered. Draw a room diagram showing doors, windows and ceiling height. If there are built-in bookcases, include those as well. Calculate the sq ft. of every wall that will be covered. Subtract areas that will not be covered (doors, windows)
Divide this figure by 57, standard roll of wallpaper/wall-covering is 57 sq ft. This gives you the appx. number of rolls needed. Note that the pattern repeat, rooms of unusual size or shape, greater than normal wastage during wallpaper installation or other special circumstances can affect the roll needs.