Number of batteries really depends on time between races. If the race has lots of entries and your only racing every hour or more, then 1 battery is fine. I typically race anywhere from 2 to 7 classes, depending on the track and only use 1 battery per vehicle because I never have an issue recharging between uses. But I also have at least 3 chargers or one quad charger.
But if there's only a few people and you're running every 15-20 mins, you might need 2.