MP3's are only a few megabytes each, say 4mb on average.
A CD-R will hold about 700mb worth of data.
700mb / 4mb = 175 tracks
Assuming an album has 14 tracks on it we have:
175 tracks / 14 tracks = 12.5 albums
OK, so thats the math behind why an MP3 CD will hold so many albums.
a CD bought from a chop or created using some kind of a 'CD creater' feature of cd burning software stores the music in a different way, where each track can be around 10x larger than their MP3 equivilents, hense only about 20 songs will fit on a disc.
Assuming you have an MP3 headunit, you to copy all your albums onto your PC into
MP3 format, if you use windows media player or itunes chances are you will get .wma or .m3u files which wont be compatable with your headunit (unless it is stated on the unit) then using some CD burning software find the option to create MP3 CDs or drag the MP3s straight onto the CD-R so that they remain in their MP3 format. If it starts converting them into some kind of other audio format you have done something wrong, cancel it & find the most basic option to put files onto the CD.
Well that was a bit of a rant but im bored at work. Hopefully that will help you though