The word is an adjective, adverb, conjunction and a pronoun. Technically it refers to two items or people so is a "dual pronoun" when used thus. Otherwise, it is not a plural.
"You" can be both singular and plural.
It is both a singular or plural noun.
Yes, the noun sheep is both singular and plural.
The plural form for physics is "physics." It remains the same in both singular and plural form.
Flesh is both singular and plural
Rabies. It is both the singular and the plural.
Parsley is both singular and plural
The indefinite pronoun 'both' is a plural form.Example: Both John and Kevin were going by bus.
The word "ground" can be both singular and plural. Singular: "The ground is wet." Plural: "The grounds of the park are well-maintained."
The plural possessive is both companies'. example: Both companies' contracts will be reviewed at the meeting.
Seals (as the plural for both the animal, and of a symbol)
Mumps is plural. But it is both singular and plural is construction.