What's the difference between "Plural" and "Mehrzahl" (and "Singular"/"Einzahl")? The dictionaries I've seen don't give much detail, except that Mehrzahl might be less common.
Plural is a Latin term, die Mehrzahl is the equivalent German term.
However, this only applies when discussing grammar, as the noun die Mehrzahl has other uses as well.
In der Mehrzahl der Fälle verlaufen Infektionen mild.
It means majority then. You cannot use Plural in those cases. The counter term is die Minderzahl, as opposed to die Einzahl when discussing the singular grammar term.