Why is the fan in a turbofan different than a propeller in a small airplane? Why do propellers typically have 2 or 3 blades, but the fan has a lot more? And why is the fan shrouded but the propeller open?

I read that a ducted fan is more efficient at very slow speeds (it generates more static thrust than an open prop), but at high speeds the drag becomes too great and you're better off with an open propeller. Which is why they use a ducted fan in hovercrafts, for instance. But if that's true, shouldn't the fan in a turbofan be open (unshrouded), considering the high speeds (higher than a propeller)? 

Also, how does the fan get away with supersonic tip speeds and the prop don't? Would the fan be more efficient if tip speeds were lower?