tire wear depends on:
time tires are spinning (better drivers tend to link/drift longer without stopping/spinning)
track surface (smooth/rough/gritty)
quality of rubber (higher grip burns off faster, but gives more smoke usually)
age of tire (even old performance tires will chunk/blow easy sometimes)
air pressure you run (too high and you wear a tire out faster)
For fairgrounds ...
I'd say bring 2 rear sets for sure and a 3rd for a just in case if you can.