As Civilwarbbq said, you can never have TOO MUCH power at a contest and when you think you DO have enough, add some more and you'll still end up being short for everything people want to plug in. Now, its often times a matter of WANTS, not necessarily NEEDS of the cookers. But, a good cooker friendly contest will do as much as they possibly can to get as much power as possible to the cookers, it just makes their lives that much easier.
I organize the Mt. Vernon, IL contest and we were lucky to have an electrician on site for the entire contest, from Friday morning through the night to Saturday about turn in time. Its amazing how you can make it through an entire contest with minimal problems, in 100 degree weather and all RV AC's running, and when people start using their electric knives at turn-in, we start blowing breakers. When you need a power fix, you often times need it NOW and having an electrician available is invaluable, if one is available.
We provided each team with their own 15AMP minimum circuit. Each 2-plug outlet box was wired to its own 15 amp breaker so teams werent in danger of tripping others' power, unless teams shared an outlet box. We also had 30AMP services and some 50AMP as well. Some of our power was temporary power laid in for the contest, that was all 15AMP stuff and it was serviced by a long run of heavy duty wire straight to the breaker box, about 50-60 yards. The site for our cook also has permanent power where the 20/30/50 amp stuff is located. When I setup the cook I always put the RV's and larger rigs on the permanent power so they had what they needed. I then put all of the teams that needed minimal power on the temporary power run. And, even in this case, I put the guys who need a bit more on the beginning of that run because by the time I get to the end of that run, even though the breaker is 15AMP, I was probably only giving them 8-9AMP of power, the rest is lost just in the long run to get that power to them.
Power is something that, in my opinion, you cant have enough of. Its like the space you provide. The more you provide each team, the more they will take. I would say that 15AMP is minimum, 20 will probably run 90% of your cookers needs, and after that, teams who need more typically will bring their own generators. Make sure the power is GOOD CLEAN power, not flakey and sporadic, and your teams will be happy.
Hope this helps.
Troy