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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.
after working with them at the local aquarium i would have to not recommend them for the general aquarist. The minimum size tank we would put the babies is was 6000g and we moved them out of there within the first month into a 10,000g tank then up to the 260,000g within 6 months. they also seemed susceptible to vision problems.
To the OP, i really would reconsider your plans to strive for a cow nose, its gonna take hundreds of thousands of dollars to build and maintain a tank of the appropriate size. Just the sheer expense of salt needed for a tank that big would be rediculous. Not trying to crush your dreams but to even start on it and not be able to follow through 100% would be an insane waste of time and money