Hi I am wondering how many par can 38 lights (often called par38's) I can run off one circuit at a time? With all the lights on simultaneously? Its for a fashion show.

By the way I'm in australia and the power outlets here are 240v and the circuits are normally on a 10 amp circuit breaker

Depends on the wattage of each lamp.

Typically you can go to 80% of the circuit for extended use (3 hours or more in the US).

Watts divided by volts equals amps.
In your case it would be 8 amps.
So a 10A circuit would be 8 amps, times 240v, equals 1920 watts continuously.