I have two batteries per car.
As all my cars take different sized packs.
At my last race meet (couple of years ago now) i could probably have managed with one, as i could recharge from a 5 minute heat in about 20-25 minutes. And that was charging at 1c.
Could possibly even have managed two heats per charge, as i get about 12 minutes driving before the battery goes. On the 2wd i almost certainly could have done the two heats, as i can get 15 mins before needing to charge.
Only downside to that is that i'd need near enough an hour to charge (at 1c)