How much music can I fit on a 500gb external hard drive?

It really depends on the quality of your music. MP3's are the most common file type and have different bit rates (the speed the file is read). The higher the bitrate the better quality but takes up more space on your harddrive. At 192kbps (a decent quality, it's in the middle of the extremes) it's roughly 1MB for 1 minutes of music. So roughly 166666 three minute songs.