Does this arc fault breaker manage one or two circuits?
That is a two-pole AFCI breaker. Two-pole breakers will disable both poles when a fault is detected. It is likely that they are connected in a "shared-neutral" configuration, which is considered a single circuit. Half of the loads (outlets, lights, etc) would be connected to L1  while the other half of the loads are connected to L2. Even though the two hot wires share only one neutral wire, they can safely handle the same amount of power as two 15 A circuits with two neutral wires. Shared neutrals reduce the overall amount of wiring used (especially useful for long runs), but slightly increases the complexity of the wiring in the house.