Wallpaper is manufactured in a variety of widths and lengths. Common roll widths are European/Metric 20 1/2 inch, American rolls 27 inches and Commercial wallpaper that is available in various widths 36", 48", 54" and 60".
Check the specifications of the wallpaper you are planning to use. It should tell you the roll width, length and the pattern (design) repeat. These plus wall length and height are the values you will need to calculate the amount of rolls you need to cover your wall.
The pattern repeat will calculate the use able amount of your roll of wall paper and will deduct a certain amount of waste you will get for having to match the design of your wallpaper for each drop. Adding a certain amount in addition to this for waste is not a bad idea. Having an extra roll will make your life much easier.
For wall openings such as windows and doors, ignore smaller sizes and calculate the total square footage of all openings combined that you wish to deduct from the area you are covering in wallpaper.