The jacket of a man’s suit can have one, two or no vents in the “tail” – the rear of the jacket.
Vents are simply tailored slits running up from the bottom of the jacket. The purpose of vents was traditionally was to allow the jacket wearer to ride a horse without the jacket bunching up, but most men won’t be taking their suit for equine exercise of that sort – the vents (or absence of them) are a stylistic feature.
Vents do still serve some purpose in enabling the jacket to keep its shape when the wearer is seated. The single vent is less useful for this than a double vent, but it does enable the jacket to retain a slimmer, straighter line – and it avoids drawing attention to a capacious rear end, which can be emphasized by the double vent.
Against that, the single vent is less forgiving when the wearer is seated (the two sides of the jacket fall apart to create unattractive gap at the base of the jacket when seen from behind) and the jacket bunches in an awkward position to one side when you put your hand in a trouser pocket.