No one knows the exact number of people that renounced their US citizenship, but the reality is that it's happening and that this number will more likely grow in the years to come.
There are various reasons for that.
The most common one is certainly high taxes and a number of laws concerning them, like FATCA.
However, apart from those financial reasons, many Americans just realized that they don't want to be part of the US system anymore - they don't share the same values, opinions and the way of life.