Why "atrio" and not "atrium"
in with the ablative atrio would specify the location something is or occurs, while with the accusative atrium would specify movement (into the atrium).
It's quite common with a "There is/ There are" sentence, to start (in Latin) with the "to be" form; but Sunt plurimae sellae in atrio wasn't accepted.
Is there a reason you couldn't write plurimae after sellae?
There is a tendency (not a rule, though!) for adjectives denoting size and quantity to precede their noun.