What do the portal pearls do? How many are there?

They transport you to wherever you want it to go. Percy, Grover, and Annabeth all search for only 3 pearls in order to save Percy's mom from Hades. You have to step on it, and say where ever you want to go. Grover stays behind, and Annabeth, Percy, and Mrs. Jackson return to Mount Olympus in order to return what was stolen: Zeus's master lightening bolt.