On one of the days in Vienna, we went on a tour of the Opera house. The tour guide told us at least 1,000 people are necessary to put on the performances. During World War II most of the building was destroyed. There were only a few rooms that had survived.
The stage is rather massive, mostly to allow the sets to be lowered to a basement floor or just removed out of the way on the side.
In the front, near the stage are very expensive seats. In the back, there are standing seats that one can purchase for very cheap the night of the event. This allows for people that do not have as much money to still attend the opera.