Actually, they both can, and are, used interchangeably, in regard to singulars or plurals.
"Contrary to popular belief, the words "Daedra" and "Aedra" are plural. It was initially meant to denote the whole kind, not a single creature. The singular form of "Daedra" would be "Daedroth", but very few known texts ever use it. In the written tradition, the word "Daedra" has somehow evolved to mean "a single specimen". I will not break this tradition here for clarity's sake, but one amendment has to be made. The word "Daedras" is essentially wrong, and I will never use it. Instead, here I will use "Daedra" for both singular and plural. You will have to do some guessing by the context. Thank you for not corrupting the Aldmeris (Elven) language!"
http://www.imperial-library.info/book_daedra/
"Sinder Velvin:
Very interesting information, Mr. Ted Peterson. Thank you. And now, a question about the words "Daedroth" and "Daedra". A Dark Elf with the profession of Savant would be called a "Dark Elf Savant", and not a "Dark Elves Savant". We use the singular "Dark Elf", not the plural "Dark Elves", in this situation. The singular of "Daedra" is "Daedroth", and yet the Princes of Oblivion are called "Daedra Princes", and not "Daedroth Princes". Is it incorrect to call the Princes of Oblivion "Daedra Princes"? Or is it incorrect to call them "Daedroth Princes"? Or are they both correct?
Ted Peterson:
Ah, the world of semantics. Short answer: Daedra is plural, Daedroth is singular. Long answer: Almost no one uses these terms correctly in Tamriel. Why? Because, just like in our own world, words change meanings out of popular usage. There is a creature simply called a Daedroth, after all: those big, bipedal, reptilian beasts that are the terror of the four corners of Tamriel. Somehow, in ages past, they were given this confusing name, probably by someone who, when asked what that creature was, gave a generic answer which was taken to be a specific one. "
http://www.imperial-library.info/interviews/tedpeterson.shtml
So officially Daedra is plural and Daedroth is singular its just most people get it wrong and the second quote is actually what I was reading when I wrote that mistake in my post. Irony. =]