The amount of lights that can be put on one circuit is determined by the maximum circuit load of the breaker compared to the amount of wattage used. The National Electrical Code recommends the circuit not to be over 80 percent of maximum load capacity.
To calculate the safe circuit load to meet NEC safety standards, multiply the light's voltage by the amperage to determine the wattage used or look at the bulb to see the wattage. Take the breaker circuit size in amperage and multiply by the rated voltage. Compare the wattage to be used to the circuit load to determine the safe amount of lights that can be put on the circuit.