Why is it called Ayin Hara (singular) shouldn't it be Anayim Raos (Plural)?
I think the answer is way more simple that that offered in the other answer: The first person to give an Ayin Hara or attempt it was Billam. He had only one functional eye. Therefore we called it Ayin Hara.
The Chida in his Sefer Pnei David says since it is the way of people to close one eye it is called Ayin Hara he only uses one eye!!