Apple quotes the capacities of the current iPod lineup as:-
shuffle – 512MB (120 songs) or 1GB (240 songs)
nano – 1GB (240 songs), 2GB (500 songs) or 4GB (1,000 songs)
5G – 30GB (7,500 songs) or 60GB (15,000 songs)
The number of music files you will be able to fit on a given iPod will depend on three factors. Mostly, it will depend on the average track length of your MP3s and on the bit rate at which they are encoded. To a lesser extent, it will also depend on if your MP3s/AACs are encoded CBR (constant bit rate) or VBR (variable bit rate). Generally speaking, all other things being equal, VBR MP3s will be smaller than CBR MP3s.
The number of MP3s that Apple quotes as fitting on each model iPod are based on the following assumptions: 128 Kbps CBR encoded AACs with an average track length of four minutes each. Updated: 12|04|06