The main breaker shows the total Amps allowable. It is usually outside by the meter or in the house inside the main panel.
Things to Consider
Under normal conditions an average home will consume between 20 - 30 amps continuously. Without physically testing with an Amp probe, there is no definite certainty of knowing what the average Amp load is.
If you have 5 empty breaker spaces in the main panel you could pull 5 dedicated circuits. 3 for the DJ/band, and two for lights and other miscellaneous needs. Each circuit would be protected by a dedicated GFCI 20 Amp breaker.
500+ feet of SJ cord #12/3 $650
5 GFCI breakers $280
Weather rated receptacles and covers $90
Labor: installation and removal. $1600
Grand Total $2680
5 20 Amp circuits is roughly 10kW of power
As this is just a one time event, the cost of renting a 10kW generator might be the smart approach financially as well as safety is concerned. The generator should come equipped with GFCI protection too but double check before the rental is finalized.
Even with a generator, the SJ cord would still require the proper weather rated male/female terminations on each end and there is always the noise concern of the generator too.
Using #12 wire I would be hesitant of running it over 200 feet. Each 100' interval might require a larger wire size. For example, 300' might require #8, 400' might require #6. I say might because basic lighting loads are not as critical/sensitive as other loads like the DJ/Band equipment were voltage quality is a concern.