But it sounds like it really varies source to source. The problem is that a lot of the time it is just an estimate. Not all self-defense uses of a gun are reported as often someone doing a criminal action is not going to self-report it. And the person with the gun may not want to report it either for fear of getting in trouble. Additionally it doesn't seem like there is really a national database that can properly catalogue and account for self-defense actions with a gun. So it seems a lot of it is based on estimates from what data they do have.
Either way that low-end estimate still seems to be quite a large number. It does make you think how many violent crimes may have been avoided because of brandishing a firearm in self-defense. It's unfortunate that it is hard to get accurate data on this.