I would strongly recommend 4+ - I spent the afternoon/evening today talking with a couple of curators at the MBA regarding ray keeping/care, who specialize in swimming rays (batrays, eagle rays, etc). The gist I got was that without a school of at least 4, they'll be constantly stressed, and inevitably die. My two have been acting very oddly, and are sickly - currently in quarantine - hoping they recover in the QT tank with no other mates (the guitarfish is a bit aggressive). I've decided to hold off on replacement until my big tank is ready, and I can stock 6+