EDIT: While the "royal we" is a common explanation for the use of plural pronouns by a single ruler, having read Dottard's answer, I believe that “us” referring to the king and his counselors is well-founded. Therefore, this answer may not hold the merit that I had initially thought, and I have relegated it to a possibility.
Short Answer: “us” displays King Artaxerxes status as ruler not that he is a plurality of persons.
The letter which ye sent unto us The plural number is used, being now become courtly for kings thus to speak of themselves: (John Gill's Exposition of the Bible)
Possibility:
Royal we and the majestic plural.
the majestic plural is a rhetorical technique used to magnify the
grandeur and authority of the one speaking. [1]
To put it simply, when a king or queen says “we,” they’re not just speaking for themselves, but for their whole country. The use of plurals is like they’re saying, “I’m the leader, and this is what we’ve decided is best for all of us.” Therefore, a king saying "we" is not indicative of a plurality of a person.
Gotquestions says,
The majestic plural, also called the royal plural, is the use of a
plural word (such as the pronoun we or us) to refer to a single
person. As a type of nosism, the majestic plural emphasizes something
or honors someone in a stylistic way. Basically, when a member of
royalty, referring to himself, says, “We” instead of “I,” he is using
the majestic plural. For example, Queen Victoria, upon hearing a
tasteless joke, is said to have replied, “We are not amused.”
The ancient Hebrews used the majestic plural, and some examples are
found in the Old Testament. But the construction is not unique to
Hebrew.
A single individual (like a ruler) might use “we,” “us,” “our,” or “ourselves” to refer to themselves collectively.
Can a plurality of persons say I, me, mine myself?
There are certain contexts where a group might be represented by a single individual using “I,” “me,” “mine,” or “myself.” However, I can think of no Biblical example of this. Today, this is often seen in legal or formal settings where one person is authorized to speak on behalf of a group.
For example, a lawyer might say, “I object, Your Honor,” in court while representing one or more clients. In this specific case, when they say, “I object,” they are acting within their role as the client’s legal voice.
