how many lights are safe?

here is a link to a handy calculator which will tell you if you are overloading a circuit on location.
to use it, change the calculate option to "Breaker size", then total the wattage rating of all the lights - for instance, if you are using four 500-watt lights your total is 2000 watts.
the result: Recommended breaker size: 20 A (20 Amps)
(this is only a picture of the calculator - the real one can be found here )

remember, if you blow out a circuit breaker or fuse - can you get to it to reset the breaker or replace the fuse?
if not, you won't have electricity for that light anymore.
EVEN WORSE, the breaker could fail to protect the circuit, and you could burn the building down.
don't do that.