Zero.
Zero lives are saved by "defensive" gun use.
Countless lives are saved by never brandishing a gun in the first place.

Defensive Gun Use Statistics
According to the National Self Defense Survey conducted by Florida State University criminologists in 1994, the rate of Defensive Gun Uses can be projected nationwide to approximately 2.5 million per year -- one Defensive Gun Use every 13 seconds.
Among 15.7% of gun defenders interviewed nationwide during The National Self Defense Survey, the defender believed that someone "almost certainly" would have died had the gun not been used for protection -- a life saved by a privately held gun about once every 1.3 minutes. (In another 14.2% cases, the defender believed someone "probably" would have died if the gun hadn't been used in defense.)
In 83.5% of these successful gun defenses, the attacker either threatened or used force first -- disproving the myth that having a gun available for defense wouldn't make any difference.
In 91.7% of these incidents the defensive use of a gun did not wound or kill the criminal attacker (and the gun defense wouldn't be called "newsworthy" by newspaper or TV news editors). In 64.2% of these gun-defense cases, the police learned of the defense, which means that the media could also find out and report on them if they chose to.