Virtually everyone is susceptible. This is a novel virus, spreading in a human population with virtually 0% immunity.
With an R_0 of 2.4, approximately 60% of the population can be expected to contract the virus within a few months, unless mitigation measures are taken.
I think Diamond Princess is a pretty good petri dish scenario for testing that hypothesis. In fact, although there are still some ongoing cases so the data is not finalized, you can normalize by passenger/crew age and reach a final infection fatality rate estimate for the whole population.
Here's a tip: it still comes out to be 5-10x higher than the seasonal flu.
most likely those more susceptible get it first
it is not clear what percentage of the population would either not get it or if they do get not get noticeably affected