Of the three defensive shootings that recently made the news here, I think the most shots fired by the intended victim was 5 or 6.
Although the retired guy fired three or four at his attacker from what I could tell. He got real lucky his only injury was ringing in the ears.
There was one incident in the news about an off-duty officer with multiple attackers where higher capacity might have been helpful if he had been lucky. One of the bad guys had an extra mag (visible in the photos) but it didn't seem like that helped him at all.
Just anecdotal stuff but with so little available data for civilian defensive shootings it doesn't hurt to try to see if specific incidents, especially in your area, follow the supposed trends.