The number of lights that are safe to use on a 15-amp circuit depends on the wattage of the lights. The safe full load rating for a circuit is no more than 80 percent of the rating for that circuit.
A 15-amp circuit is overloaded at 1,800 watts, the result of 120 watts times 15 amps. It is at 80 percent load at 1440 watts. This means that it is safe to connect 14 lights with a rating of 100 watts each to the circuit, or 22 recessed 65-watt lights. This assumes that there are no other devices connected to that circuit.