What is happening is that you are recording a data cd when you put that many mp3s on one cd. When you have twenty on there it sounds like your software is making an audio cd.
Usually, you choose the type of cd you want to make when you start the burning prog. To play in a regular cd player, you're going to have to make a regualr audio, or cda, cd and be able to fit 74 minutes (650mb) or 80 minutes (700mb) of music.
If you make a data cd with mp3s on it, you'll be able to fit around 650 or 700 minutes worth of music (figuring about 1mb per minute, but this varies depending on bitrate) but you'll only be able to play it on computers or cd players that support mp3 playback...
When all else fails, read the instructions.
Microsoft Knowledge Base