Different Species

There are 18 species of penguins in the world. 13 of these penguins have declining population, and 5 of them are considered endangered.
 The names of the 18 species:
Adelie
African Penguin
Chinstrap Penguin
Emperor Penguin
Erect-Crested Penguin
Fiordland Penguin
Galapagos Penguin
Gentoo Penguin
Humboldt Penguin
King Penguin
Little Fairy Penguin
Macaroni Penguin
Magellanic Penguin
Northern Rockhopper Penguin
Royal Penguin
Snares Island Penguin
Southern Rockhopper Penguin
Yellow-Eyed Penguin

I wonder if these are trully the only penguins out there. I know explorers have not fully explored every inch of this world of ours. So how do we realy know if there are only 18 different kinds of penguins. I fully believe we have not discovered every kind of penguins out there, just like there are other species of different animals that have not been discovered. Some are being discovered every month.