I don't think it has a plural. It's not a noun, but either an adjective ("sorry fate") or, when apologizing to someone, an adjective that becomes an interjection, which generally don't have plurals. You'd have to use apologies.

Sorry for the lecture!

Oh, and I think SD's interpretation is probably correct. When people are trying much harder than they acknowledge to themselves and they apologize to me for some perceived laziness, I often say something like "you have nothing to be sorry for."