The reason I recommend at least 6 fish is twofold. 1) You may have to remove at least one fish if sorority living turns out to be impossible (some fish are just too aggressive for it). 2) Bettas are territorial. Having too few fish allows for individual rivalries to develop, and this could result in one fishing picking on another all the time. With more fish, there is more distraction so as to prevent any one fish from getting too focused on another.