For 15 amps: Lets suppose that all your lights are 60W: 120Volts * 15Amps = 1800 Watts; so from there a regular 15 amps circuits has 1800W to spare therefore: 1800/60 = 30 lights at 60 W each, or 1800/100= 18 lights at 100 Watts.

Amps=Watts/Volts 100W/120V=.83Amps