On: Obama presents NSA reforms with plan to end government storage of call data

The Inspector General, in this case of the NSA. Basically an internal watchdog. They were aware of at least 12 cases of abuse according to the article.

How did they come to be aware of them? The information was volunteered or investigations comissioned at the behest of victims.

So... follow through with this. A significant portion of the NSA are abusing their powers to spy on average people?

An unknown amount of the NSA are.