You actually only need one
It all depends what you are going to install and if you are going to install from sources. Do you have a good internet connection? What are you going to use the machine for? The cds have the STABLE version, that as todays writing is debian woody. If you are going to use if for home, you probably will want to use newer software, some testing or unstable stuff like a newer gnome or kde. So install the minimum posible and then install all the programs you want via apt. If you dont have broadband internet, then you can ask a friend for an apt-zip on a cd with newer software.