If you're in the position where you can have any size tank then obviously you could fill it with as many Oscars as the tank permits. Obviously, most of us can only house small aquariums at home. If your aquarium is no bigger than 75 gallons then that limits you to one adult Oscar. Once you get to 100 gallons + then you can start thinking about having two Oscars. Three or four Oscars would require a sizeable tank well in excess of 200 gallons. A tank of this size would probably need to be custom-made for you. However it's vitally important that you remember that the size of the aquarium is only half of the equation, filtration would need to be increased considerably once you start keeping two or more Oscars.