You usually get 3 drops and a bit from each roll. Older houses usually have higher ceilings, but you should still get three. Then just go round your room and count the drops you already have on the walls. You will need to allow extra in case you make a mistake some where, and make sure you can get more of the same paper from the store if you need it.
Be sure to check your batch numbers on the rolls before you buy, otherwise the paper could be slightly different.
It's not that difficult once you get the hang of it.
The main thing is making sure you use a plumb line to get the first piece up straight.
