But there's this line Spock Prime spoke:
"As I began my return trip I was intercepted.
- He called himself Nero,
- Last of the Romulan Empire."
That seems to indicate that most of the Romulans perished and the Emprire itself is no more.
Nero was hardly a detached, rational commentator. His self-applied epithet can easily be interpreted as hyperbole, an expression of his feelings rather than of the objective reality.
After all, the film gave the impression that it was only hours after the destruction of Romulus. Information is always unclear at a time like that, and rumors can propagate out of control. I remember going online on 9/11/01 and seeing hysterical posts from people (well, one person, at least) convinced that the falling of the World Trade Center had killed millions of people. It's possible that Nero had heard exaggerated accounts of exactly how much destruction had been wrought Empire-wide.