For a 12ft by 10ft room with 9ft high ceiling, you'll need about 8 rolls (allowing for one door and a medium sized window). Approximately ... You can calculate properly here http://www.wallpaperdirect.com/wallpaper-calculator.php?infeet=1&rowid=
Knowing that my neighbour had recently papered an identical room in his house – I asked how many rolls he had purchased for the job.
After I had finished papering the room, I advised my neighbour that I had 3 complete unused rolls left over – he replied that he also had that number remaining.