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