Right, that's why I criticized "number of particles" as a metric, not the study in general. The point is that "pieces of plastic below 5mm in size" does not have a lower bound, and indeed the journal article discusses pieces that are 300 microns across, overwhelmingly most of which are less than 100 microns across (meaning many thousands of times smaller in mass than suggested by your phrase). This is getting close to metric I already said
would be a good one, i.e., ppm, or some other similar metric along with an estimate of the size of harms at this level.
For that, you'll have to actually read the paper (please share!) or wait for non-terrible science journalism -- don't hold your breath!