So far as I am aware, there is only a single Shielder in existence. I don't know why, but the Demi-Servant Mash Kyrielight is the only one Chaldea knows of. She's more than capable of fighting, she just doesn't have any inherent advantages or disadvantages like the other Classes.

Shielder isn't the only Extra Class, though. There's also Ruler, Avenger, Alter Ego, Moon Cancer, Foreigner, and Pretender.