How many cans on one 15 amp breaker?

Assuming the can lights is all that will be on this 15 amp 120 volt circuit and each one has no more than a 75 watt bulb installed you can install up to 19. This is determined by multiplying the amps x the voltage to get the wattage. 15 x 120 = 1800. Then multiply the 1800 x 80% and you get 1440 which places an 80% load on the circuit which is all you safely want. You then divide the 1440 by 75 which is the wattage of each bulb and you get 19.2. You could add more if you use fluorescent bulbs but someone could come along later and install incandescent bulbs and overload the circuit. So stick with no more that 19 to be safe. Remember this is a 15 amp ciruit so you must use AWG # 14 wire and not #12. If you use AWG #12 and a 20 amp breaker you can have 25 can lights with 75 watt bulbs on the circuit.