Basically your total amperage will equal your watts divinded by the volts.
AMPS = WATTS/ VOLTS
So if you were using a 20 amp breaker, 50 watt light bulb and 120 volt line, you would use the following. Remember, you want to use only 80% of the breaker capacity just to be safe and save worries later down the line.
Amps per light = 50 watt bulb / 120 volt
Each light would use .416 Amps.
80% of the 20 amp breaker would be 16 amps. Divide the 16 amp breaker by
.416 amps per light.
20 amps / .4545 amps per light = 38 lights.
Theoretically, you could use 38 lights.