I seriously doubt the number of attacks thwarted is classified. She didn't reveal any details here.
However, blabbing about that arrest was probably a screw-up.
It was the reporter who chose to focus almost exclusively on this one small portion of her entire speech. He left right after Harris' speech was over. It's fair to say that the reporter is well aware of the limitations that exist in providing details of such events, and why the majority of terror threats that have been found and eliminated in the CONUS are not reported. But by making an issue of it, the media seeks to denigrate Harris' statements, and therefore her position. No surprise there.