Well it turns out I'm wrong about this, there are some studies with numbers but they're pretty rare:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3655081/
> Twenty-two percent of all subjects with isolated coronavirus were hospitalized, but in the population aged ≥60 years this increased to 73%. While our study design precludes the determination of exact hospitalization rates as a result of coronavirus infections, the hospitalization rates were similar to those for influenza.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4181476/
> The high proportion of non-influenza infection in this study (72%) underscores the importance of non-influenza viral infection in contributing to severe illness necessitating hospital and ICU admission in adults, as has been found by others.
