I'm not going to follow the link that you gave and I strongly suggest using BitTorrent instead of a mirror, but the answer to the question is that there should be about 14 CD's in a full set. These include lots of extra software and in general, as long as you will have an Internet connection later, you'll want CD 1 and to install from that. If you do want any of the packages on later CDs you can download just what you need over the Internet with the Debian package system. On the other hand, if you are about to move away from having Internet access, the more CDs you get the more packages you will have access to. It is said that the packages on the CDs are sorted in order of how many people actually install them, and that even if you didn't have Internet access just the first few CDs would suit most users. My advice would be to start with just the first CD and only get others if you find a reason that you need them.