There is a small efficiency loss as you add blades to prop (basically losses due to interference with the wake from the leading blade) but we're not talking big numbers here. The main difference when adding blades is you can do the same work (i.e. move the same amount of air) with a smaller radius prop.
The big magic with a turbofan is the way the inlet slows the airflow before it hits the fan - again see the discussion in the other thread I reference.