We as Humans can only hear so much of the sound spectrum. There is no point in sound that we as humans cannot hear being in a sound file (song is this case). An MP3 has all of the sound that we as Humans can hear in it. That is why the file size is small. All of that "other" sound that we cannot hear only increases the file size and as we cannot hear it, there is no point in it being there.
So an MP3 is a small file with all of the sound that we can hear.
How big an MP3 file is depends upon the encoding.
For example an encoded file at 128kbps would sound far superior to a file that is encoded at 64kbps. However at 128kbps, a file would [say] have a file size of 4MB but a file that is encoded at 64kbps would be [say] 2MB (a rough example, lenght of a song also comes into play).
So you can have either a small file but with reduced sound quality or a large file but with increased sound quality.
128kbps is generally the norm for MP3s...They give good files sizes, with sound quality to match.
When you save audio tracks using Realplayer they are saved, as *.wav, *.mp3 or Reals own media type.
To make RP save to *.mp3 when you insert your CD, click "Save Tracks" and then click "Change Settings..." and you can select mp3 from there. But the quality is limited to 96kbps... You have to buy the plugin in for better quality.
However there are alternatives that can do the samething for free...
http://www.mthreedev.com/
http://www.dbpoweramp.c
om/dmc.htm
Search
on www.google.com for many others...
As to how many can fit on 20GB = Thousands!