It is tempting to buy a couple of juvenile Oscars when they all look but just 3 inches long in the pet shop. Their playful ways with one another may make you think that they will all get along when you bring them back home to that tank you've set up for them. I would think twice if I were you.
First things first. You have to realise that the Oscar fish can actually grow up to a foot or longer in aquarium conditions and for up to 10 years if not longer. That little juvenile which is only 3 inches long can grow up to four times its size (and create 4 times as much mess!). You are thus looking at a long term relationship with the cichlid called Oscar that you are so tempted to buy.
The answer to how many oscar fish one should keep depends on how big a tank you are setting up and whether you are prepared to keep them for that many years. And please don't do the environment-hazard thing of throwing them into the nearest lake because the Oscars can be belligerent enough to cause havoc to the natural fish ecosystem in that lake. We wouldn't want that, would we?
I would use a rule of thumb of providing an aquarium size at least 3 times the length of the maximum size of the Oscar fish if I were to rear a single Oscar. That means, the tank should be of at least 3 feet length to make the Oscar comfortable and to allow it to grow to its maximum potential. If you intend to rear two, you will have to provide a much bigger tank for obvious reasons - and also giving consideration to the fact that these are territorial creatures.