YouTube boss Susan Wojcicki said the "significant" increase in numbers would help clamp down on offenders.
YouTube has been heavily criticised for failing to prevent predatory comments targeting children and the widespread availability of terrorist propaganda.
Ms Wojcicki said the company had already toughened its policies, including taking "aggressive action" against offensive comments, and was testing new systems to help fight emerging threats.
The site is also developing machine-learning technology to automatically flag up inappropriate content, she said.
Last month, investigations found paedophiles were posting indecent comments on videos of youngsters, evading discovery through flaws in YouTube's reporting system.
Adverts for major brands were appearing alongside some of the videos, which led to several big brands, including Mars and Adidas, pulling ads from the site.