Cownose rays live in large schools and move constantly - they do not sit on the bottom and hide like stingrays. They are a mobile species. It can be done, but it is most likely far outside the scope of most hobbyists ability and resources.
5+ generally speaking from what I have seen. A group of that size might be doable in a tank of around 20x8 feet or so, but that would be absolute bare minimum. They cruise constantly, as with most eagle rays.