The report from the Chinese CDC said 1.2% of patients were asymptomatic. (That's counted separately from the 80% who have mild cases.) A higher portion of asymptomatic cases was found on the Diamond Princess cruise ship, where 380 of the 691 people tested positive but showed no symptoms.

That means the real number of asymptomatic patients could be higher, since patients who don't realize they're sick don't seek testing or medical care.
https://www.businessinsider.com/what-coronavirus-mild-symptoms-are-fever-2020-3#according-to-early-studies-12-of-covid-19-cases-are-asymptomatic-which-means-those-patients-experience-no-symptoms-at-allhowever-they-may-still-spread-the-virus-7