That would be because you're burning full audio cd's, not data cd's. MP3 files are ALOT smaller than generic audio files and can be burnt as data files. If you're burning mp3 as a data cd you'd be able to fit 100-150 songs per CD. But the cd would then only play on machines that play mps files.
MP3 on the whole are around 3-4MB per song unless you have a very high quality version i.e. 320kpbs which are then between 7-10MB each, still be able to fit at least 70 songs still as long as they're burnt as .mp3 and not audio.
It's difficult to get used to but if you want to add your songs off CD's or your PC just go to "File > Add Folder to library" or "File > Add file to library", with folder being a folder full of songs and file being a single song.
Once you do that those files should appear in your library and you should see them listed. To get these songs onto your ipod you can either sync the library or highlight the songs you want to transfer by holding shift whilst clicking on the songs and then dragging them over to where it says "Missus* Ipod" from the menu's on the left. (* being your own ipod name)
I brought my husband an iPod shuffle for xmas I think. It was about £30. Fantastic if u only want it for exercise as the are very small and clip onto clothing