N95 respirators have a pore size on the order of 100-300 nm. Viral particles can be a lot smaller than that (5-10nm), but this coronavirus seems to have a 80-90nm diameter. That means a respirator might be able to stop the particles IF worn perfectly (if you have stubble or a beard, you're already screwed). They will however stop droplets from someone sneezing or coughing, again IF worn properly. Medical professionals get fitted regularly to ensure proper fit.

Masks are only effective in a fairly immediate radius: someone coughs, that droplet goes a little ways then falls on a surface. Medical professionals are in that immediate radius all the time, that's why they need the masks. People are usually not sneezing directly on each other, so that's less of an issue. What IS an issue is the viral particle that is now on a surface. You touch that, move your hand to your face (perhaps to adjust your mask) and bingo you're infected. Washing your hands obsessively is WAY more useful than any mask.

Most of the time in medicine masks are used to stop the medical professional from contaminating the patient, not the other way around (that's what you see in surgery). If you're with someone sick, it would be good to give them a mask, they'll spread fewer particles that way.

TLDR: Masks are minimally useful for the broader public, very useful for those directly in harm's way. Don't buy masks unless you're sick, or someone close to you is sick. Let the supply chain work for medical professionals who actually need them.