in 2016 Google took down 1.7 billion ads that violated its advertising policies

"We have a strict set of policies that govern the types of ads we do and don’t allow on Google in order to protect people from misleading, inappropriate, or harmful ads. And we have a team of engineers, policy experts, product managers and others who are waging a daily fight against bad actors. Over the years, this commitment has made the web a better place for you—and a worse place for those who seek to abuse advertising systems for their own gain.
In 2016, we took down 1.7 billion ads that violated our advertising policies, more than double the amount of bad ads we took down in 2015. If you spent one second taking down each of those bad ads, it’d take you more than 50 years to finish. But our technology is built to work much faster."

Source:  Blog post from Google, 25th January 2017