The word ‘whom’ is not plural. It is actually the objective case of ‘who’ and is used to refer to individuals in the context of being the object of a verb or preposition.
For instance, you might say, ‘To whom did you give the book?’ Here, ‘whom’ refers to a single person who is receiving the book.
However, you can use ‘whom’ to refer to multiple people in a context where it is appropriate, such as in a sentence like, ‘To whom it may concern.’ Though ‘whom’ itself does not change to indicate plurality, it can still apply to more than one person based on the sentence’s structure. Thus, while ‘whom’ is not inherently plural, it can be used in contexts that involve multiple individuals.