Bear in mind one thing. 128 is almost cd quality, unless you have the greatest headphones in the entire world.. it is not optimal for a portable player with tiny earphones.
For my brother's player, I encoded them at 80kbps. 21 songs came to 54mb, roughly 90mins of play. You should be able to squeeze in almost 2hours of music. They still sound fine to me. Upto you though..
The lower you encode, the more mp3's you'll fit on, the worse they sound. And obviously the opposite is true.
Oh, I think the muvo plays WMA too, which should be able to fit even more songs on..