how many songs can an iphone 3gs 8gb hold

It depends how long the songs are, how many apps you have, how much data those apps use, how large the apps are, how much email you keep on the phone, how many text messages you keep on the phone, etc. For typical popular music it is about 10 MB per song. So if you have no apps, no email, don't save text messages you could have about 600 songs in the 6 GB available on an 8 GB device.