Quantity of races is dependant upon numbers and also the number of active volunteers to build the track
Usually we aim to give 4 rounds and a final.
Most cars these days can do 2 runs on one charge, depending on the capacity of the battery and the motor used. I would advise 3 packs if you do not wish to recharge at the meeting, but if you have a charger and power supply, I see no reason why 2 packs would not be enough (1 charging whilst the other is being used)