#1 Only one title has ever changed hands in Clash of Champions history
Interestingly the United States title is also the only title to ever change hands in the history of Clash of Champions with Rusev and Baron Corbin unable to successfully mount title defences for the US Title at the PPV. This is despite every title on the brand being defended at the show.
In 2016, despite every other title on RAW other than the United States Title, it was successfully defended, with the Universal Title, RAW Tag Team Titles, RAW Women's Title and Cruiserweight Titles all staying in the same place. And in 2017 the SmackDown Women's Champion, WWE Champion and SmackDown LIVE Tag-Team Champions all retained.
As a further aside, the prestigious Intercontinental Title has never been defended at a Clash of Champions, as it managed to avoid both the 2016 and 2017 PPVs by switching brands in between.