The way torrenting works is people distribute a file, say Ubuntu Linux by using their own Internet connection at home.
Most home connections are what is called ADSL / Asynchronous - which means they download faster than they upload. SDSL / Synchronous mean the rate is split evenly.
The number of people who are distributing will have an effect on speed.
If you have say 8Mb speed (as advertised!) you might get just 6Mb down / 2Mb up as the companies cannot say for technical reasons how fast you will be able to get.
To check you get what you pay for try this free tool:
http://speedtest.net
It will allow you to see what your connection is capable of. My 20Mb connection says I can get 4Mb down / 0.5 Mb up (at the time of answering)
With that, you would need 8 people to give you 4 Mb download speed. Some torrents such as science projects or research databases may be online at different times of the day, which can also help or hinder your speed.
If you have Windows, it has some settings which are never optimized for connecting to the Internet. Automatic setting tools are available at:
http://www.speedguide.net/downloads.php
Which will allow your windows version to work to standards later than 1995 (written 2010 !!!) it changes some half open connections to fully open, plus increases the number of connections from 2-25 upto the hundreds.