A schooling fish by design
Black skirt tetras come from slow South American waters where they live in loose schools, and the schooling instinct is not decoration - it is their security system and their social structure in one. In a group, black skirts spend their day in a shifting mid-water cloud, sparring mildly among themselves, displaying their tall black fins and settling their own pecking order. Watched closely, a settled school is busy but peaceful. The fish was never built to be a solitary pet, and most of the behaviour problems attached to its name trace back to groups that were simply too small to let it behave normally.
Six is the floor, eight to ten is the answer
Six black skirt tetras is the minimum that lets the school function, and if your tank has the room, eight to ten is the better number to actually aim for. The difference shows up in the nipping this species is known for. In a small group, a restless black skirt directs its energy outward, at the trailing fins of angelfish, guppies, bettas and other slow, flowing-finned tankmates. In a full school, that same energy is absorbed by the group itself: there is always a rival to display at, a neighbour to chase off a spot, a cloud to rejoin. Bigger groups also look like what they are - the black skirts hold together and move as one dark, flickering shape rather than scattering as nervous individuals.
What the group means for the tank
Numbers are only half the decision; the other half is whether the tank suits them. Black skirts are deeper-bodied and more substantial than the slender tetras, active across the middle of the tank, and a school of eight to ten wants a tank with real swimming length rather than a tall, narrow column. Remember that each added fish adds to the load on the filter, so build the group in a tank that is filtered for it, and think twice before pairing a big school with very slow, long-finned tankmates even at the right numbers - temperament improves with group size, but the temptation of a flowing fin never entirely disappears. Plants and open water in balance suit them: cover at the back and sides, room to school in front.
Building the group
Buy the full group together where you can. Adding the whole school at once lets the pecking order form a single time, in a group finding its feet together, rather than restarting with every newcomer. If you are topping up an existing pair or trio to a proper school, add at least three or four at once so no single new fish takes the whole group's attention. Over the first days, watch the fins of tankmates and the school itself: mild chasing among the black skirts is normal settling, while a fish hiding constantly or fins being shredded means the balance is wrong and the group usually needs to be bigger, not smaller. If you want the school size checked against your tank before the fish come home, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.