Errrr, depends on the bit rate they are encoded in and how long the songs are.
if they are encoded in 128Kbs about 40 full albums, (it's easier to say albums because they are always roughly the same length whereas a single track can be anything from 2 minutes to over an hour)
I really wouldn't recommend encoding in 128Kbs, I do all mine in 320Kbs and there is a noticeable difference in quality.
If you are planning on plugging the stick into anything like a car stereo to play your music it's also worth checking if there is any limits on things like size of stick, folder structure, amount of folders/files the devices will recognise.