I saw something a couple days ago that 25-50% may exhibit no symptoms. I would have to go back and look at what study that came from- may have been a report based on 2 different ones.
Given the lack of available/timely testing, and how highly contagious this virus is, it may end up that the majority of us will be exposed to it in the coming weeks/months, and those who exhibit no/very mild symptoms will simply never know they had it. The social distancing/use of masks, etc are the key to preventing the non-symptomatic from spreading it. Plenty of uncertainties with modeling(as we all know here). Just my unscientific, pragmatic thoughts.