That's a tough question to answer accurately, since songs vary in length and audio quality, which means different songs in MP3 format take up different amounts of space on your MP3 player. The best we can do is estimate, but a good average file size for MP3s is 4MB.
Since a GB is roughly equal to 1,000MB (1,024 actually, but let's stick with round numbers!), you can fit about 250 songs at 4MB per GB of storage.
So, if you have 8GB free on your MP3 player, that will hold about:
8 x 250 = 2,000 songs. Roughly.